Good question. As a mentor, you need to know which areas need to be worked through in order to gain as complete a picture as possible of the state of research. I'll take an area that I used to know quite well: psychology. One examines the large review articles and the given literature. Of course you should know something about the history of the subject, and should not rely on online research, e.g. with World Cut, many important things from the past are not in the "system"!

Yes i agree with u @

Step 1: Focus Your Research Area

Before you start trying to identify gaps in the literature, you need to figure out what your area of interest is, and then focus and narrow that research area. If you don't narrow down your initial research area of interest, you'll end up wanting to research everything. You'll overwhelm yourself with all the research gaps you find because there are still a lot of unanswered research questions out there. 

• Do some exploratory research on your broad research idea in your course textbook, class notes, in meta-analysis, systematic, and literature reviews, and PsycINFO Thesaurus to identify more specific issues and arguments in your research area and possible relationships between them.
• Read ebooks to get the "big picture" about the research area you're interested in studying. Books and ebooks provide detailed information on your research area, put your research area in context, provide summaries of research, and help you identify major themes and relationships for your study.
• Ask your advisors and other faculty about possible topics or issues within your research area of interest. That being said, you're going to spend over a year immersed in work on your thesis, so make sure you choose issues because you find them deeply interesting, not just because your advisor recommended them.

Step 2: Read, Read, and then Read Some More

Read (a lot of) research articles: this is going to be time-demanding, but you really do need to read through a lot of research articles in your research area to become an expert in it. That being said, what you use from the articles that you read should relate directly back to your focused research questions and hypothesis. Don't waste your time getting sidetracked by issues that don't relate to your research questions and hypothesis.

• Go to Start Finding Sources, Search Databases, and Browse Journals to find journal articles for your research area
• Pay close attention to Introductions, in which authors explain why their research is important, and Suggestions for Future Research, in which authors point readers to areas which lack investigation or need future examination

Follow the research trails of seminal articles and authors using Web of Science and Scopus:

• Articles
  • In Scopus, click on Document Search, enter the article title, click on the article title in the list of search hits, then click on View all ~ citing documents link in the right sidebar for a list of articles that have cited this article
  • In Web of Science, enter the article title and choose Title from the right drop down menu, then click on the Times Cited number next to the article to see a list of articles that have cited this article
• Authors
  • In Scopus, click on Author Search, enter the last name and first initial(s) of the author, click on the author's name in the list of search hits, then click on Cited By ~ documents for a list of articles that have cited this author
  • In Web of Science, enter the author name and choose Author from the right drop down menu, then click on the Times Cited number next to each article to see a list of articles that have cited this author's article

Read meta-analyses, literature reviews, and systematic reviews: these papers delve deep into the literature, examining the trends and changes over a long period of time in your research area and summaries of previous research findings.

• In PsycINFO, click on literature review, systematic review, and meta analysis under the Methodologies heading in the sidebar to the right of the list of search hits 
• In CINAHL, add systematic reviews to your search 
• In Web Of Science, check the box beside Review under the Document Type heading in the sidebar to the right of the list of search hits

Step 3: Map out the Literature:

Keep track of what the authors told you and the questions that occur to you whenever you read anything - an article, a book, a book chapter, a dissertation, etc. This will also help you write your thesis introduction later on and help you avoid unconscious plagiarism.

• Use mind maps, tables, charts, pictures, post-it notes to map out the literature, whatever works for you. 
• Research each of your questions to see if there are people out there who had the same questions and found answers to them
• Science Direct, Web of Science, and Wiley Online Library databases help you follow the research trail by listing articles that have since cited the research article you're reading

If you find don't find any answers to one of your questions, you've probably found a research gap from which you can develop a thesis hypothesis and experimental project. Get feedback from your advisors before you get too carried away, though!

Source: https://guides.umd.umich.edu/c.php?g=529423&p=3621573

Thank you all, am going to use all your ideas in my talk. Anything else we are leaving out. Colleagues will tackle other aspects my focus will be only be on research gaps. I appreciate your feedback, our talk will be towards the end of the week. I welcome more ideas on coming up with research gaps. Thank you

A new one are citations on research gate profiled on google scholar? or it takes time till google scholar finds the citation on its own on other links?

Especially useful is the time wasted. They are needed to answer questions like these with interest.

it is even sometimes difficult for the instructors and researchers to find a gap; therefore, the mentor him/herself should read the available sources on a certain topic and then recommend some topics for the students and ask them to find if they are covered or not. in this case, the students can think broadly and discover the gap or at least the area which can be further discussed and expanded.

Thank you Bakhtiar, am adding this one. Its true we can suggest an area when they take forever to get something or when they seem absolutely clueless on where to start, after we have made clear they need to choose an area of interest to them, since once they own the idea, it will give them motivation when they feel exhausted or drained to continue working on it.

After reviewing their work and helping them narrow, I myself check for recent research using Google Scholar and/or Harvard Library's catalogue or our own university's data base and collection (which allows a one-time search of many sources). I usually find 1-4 compelling sources that get my students more focused. I figure most times they can take it from there. Sometimes it's that extra help into research that gets more focus and ideas about finding more.

I used your input in the postgraduate seminar and am glad you each of you participated in one way or the other. The students benefited from global advise. Blessings