One thing that I would add to all of this good advice is that there are different levels of testing.  When someone first learns a new bit of grammar in a foreign language, they could probably complete a worksheet on it, but it is much more challenging to use it in their own spoken grammar several weeks down the road.  Also, some learners may comprehend the new grammatical structure well before they produce it and for others, it might be the opposite.  For that reason, I would recommend following up on grammatical structures that are taught in several ways (orally, written, worksheets, comprehension, etc.) for several weeks and months after teaching them.

You probably want to use a Pre-test Post-test methodology.  But many studies I have seen are not persuasive because they have students use an experimental technique for a short time, like two weeks, and then immediately test to see if they remember what they learned.  More important is whether they remember a month later or three months later, i.e. whether they retain what they have learned for the long term.

I am not exactly sure what you mean by "grammar points" but this is my general advice about measuring retention of learning.

If you develop a rubric for the levels of retention for each specific lesson/skill previously taught, you could use that rubric to evaluate periodic authentic performance assessments.  

Have students provide you with a writing sample (done in class for a specific time based on a writing prompt you provide). If this is done as a class "warm up" each class (or 1x per week or 1x per month) you will have a number of data points that are consistently evaluated by your rubric across the time period of learning.  Because the long-term goal is to use grammar "in situ" , then this type of authentic performance assessment will allow you to see if what was taught actually transferred to authentic usse. By keeping the assessment time short (5 mins), the length of the writing will not be overwhelming to assess later. 

You could also assess spoken use of grammer the same way using a short video clip assessment of the students answering a predetermined question  (but would have to ask the students to come to a testing center or "meet" individually on-line- through Skype- to control for anyone who may want to "practice" and tape only the best sample. You can use the same rubric you created for your written grammar assessment.

I would make sure everyone is given the SAME writing prompts for everyone and use questions that are opinion based (versus answering a factual-based prompt- which can be biased if someone has expertise in that area- as it will bias the use of grammar due to more cognitive load for those answering w/out the benefit of the actual knowledge). I like to use those question books or deck of table-talk cards people buy for parties as ice-breakers. 

 Dear Professor Marek,

Thank you very much for your kind advice. I can certainly apply it to my grammar retention assessment. 

Best regards,

Cameen

Dear Trish Walcher,

Thank you very much for your kind advice. I'll definitely try this method in my experiment. 

Best regards,

Cameen

I think, some teachers teach deductive grammar instruction whereas others teach inductive instruction at secondary and high schools stages.

When you teach induction form of grammar, you should apply teacher-fronted instruction and decontextualized method. For example, when your students practice some grammar examples. You should draw the grammar wall and make certain examples of the sentences used in daily life. Then, you should allow students to continue applying the rules. This  would benefit  students to practice grammar-wall-based learning which would emphasize the importance in writing skills also. In results, students would emphasize on decontextualized and rule-provided grammar instruction, and they  would spend a great deal of time learning grammar with complete retention.

As far as I know, most of the research on retention of skills (used naturally with automaticity) for both typical students and dyslexic students learning their own native language OR in a second language is based on contextualized learning (and thus assessment). While direct teaching and practice of decontextualized skills in an organized, systematic manner does help students develop a "bank" of information to use, the ultimate goal is to USE language for real communication. Grammar is part of living language and there is little research (of which I am aware)  that emphasis of predominately rote teaching of decontextualized skills retains that information in situ. I do not think anyone teaching any language to any level of learner (of any age) has the goal of retaining memorized rules versus use of language skills to accurately communicate a thought using correct grammar (vocabulary and inflection).  Perhaps, if the intention is simply to pass a grammar test, then yes, rote skill and drill will help retain that information. But, as a literature review of learning development and instructional design (as well as child development) would demonstrate, THE WAY ANY SUBJECT IS LEARNED -encoded as it is rehearsed and retrieved when practiced- is the WAY IT WILL BE  USED. If the goal is to retrieve isolated facts in a practiced test-recall practice, then teaching to respond to decontexualized grammar rule recall requests is the way to go- and will work very effectively to serve that purpose. If the goal is to learn to effectively communicate in an authentic manner USING grammar that is able to increase the capacity to be understood in that language, then one needs to practice the use of grammar within real world, authentic situations. Like most things, there is no one "right" way, but depends on ones INTENT/ GOAL. Complete retention of isolated rules is great for tests but when trying to communicate, TRANSFER is often the greatest problem noted within the research of all learning (including language).  

One thing that I would add to all of this good advice is that there are different levels of testing.  When someone first learns a new bit of grammar in a foreign language, they could probably complete a worksheet on it, but it is much more challenging to use it in their own spoken grammar several weeks down the road.  Also, some learners may comprehend the new grammatical structure well before they produce it and for others, it might be the opposite.  For that reason, I would recommend following up on grammatical structures that are taught in several ways (orally, written, worksheets, comprehension, etc.) for several weeks and months after teaching them.

Two sides to your question: the one about explicit vs implicit is I guess an important one because very often we do not see learning from the learner's viewpoint, and we impose a way to learn which is totally alien to them. Besides, in certain cultures, I am Muslim, repetitive exercises are more than advised to make sure that it is learn and that nothing is lost. Thus, working on the explicit side might generate better results. People are ashamed to speak about rote learning. I went through this hardship when I was young, but grammar is still here decades after I learnt it at high school. And one is talking about being efficient! I believe it is Wilkins who stated that grammar should be taught in L1 to make sure that equivalences will be made between the two codes. As for evaluating their retention capacity short quizzes might do, pair-work could also be used to evaluate one's and the peers' knowledge of grammar.

I think that when learning basic rules and procedures, we certainly want to practice to automaticity, but practice of basic skills eventually needs to be used within authentic contexts- or one runs the risk of perfectly learning isolated information (what is commonly referred to as "inert" information). Grammatical rules are useless  in real life situations, if one is unable to both receptively understand (in reading and listening) as well as use expressively (in speaking and writing). Again, if the point is to pass tests where assessment is basic repeating back of rote of controlled, isolated information then practicing that way will certainly be effective. On the other hand, I have had many friends pass the TOFL here in the states, but have great difficulty expressing themselves well enough to teach native English speakers here nor do well with expressing complex understanding (not because they do not understand the topics they are teaching or what is being requested, but simply because they need more practice with natural language use. The more a student was spoken to and spoke in English, the better they were in overall communication. The more they had learned English as an isolated skill that was only practiced in a class and only tested, the less effective. Transfer of knowledge to real world use, especially when we are talking about a "skill" is directly related to time spent practicing in real world situations. Direct teaching and practice is especially important in the beginning stages, no one would argue that, but authentic practice is what bridges the "gap" from theory to practice, once the basics are automatic. So, if we are talking about very novice learners (who may simply be taking a course to fulfill a basic language course requirement) I could see focusing on more direct, skills based instruction. But, if we are talking about the instructional design of a learner across their career (learning to speak, read, and write for communicative purposes with natives of said language), then for transfer to occur, you need to teach (and assess) doing those activities. 

In response to the comment of thinking about learners- I am not sure if it is cultural-as I have heard other US teachers talk about teaching their subject matter instead of teaching their students, but that difference in emphasis is telling. If you are teaching a subject, you are teaching to a thing- rather you should teach students a subject, as they are who are the persons who have to make sense of what teachers are trying to communicate. We already know what we mean (most times), so it is our job to communicate at their level of understanding. Without the ability to take the learners' perspective the likelihood of motivating your students to learn something new (adapt) will be thwarted. 

Retention on grammar point has a very basic end of our mind & memory .To judge & evaluate the students from grammar points is the very basic principle rules & regulation of grammar covering the rules of grammar ,tenses ,spelling,each in the initial stage make the good understanding of English with its retention memory .