Capture pollinators and wash their bodies in aniline blue drops placed on a glass slide and observe under microscope.  Count the number of pollen grains present  and accordingly evaluate their pollen carrying efficiency.  Further, in case of honey bees, do not take pollen from their corbiculae (because there will be loads of pollen and hence avoid corbiculae).  Count only pollen grains present on other body parts of these honey bees.

One method we have used in the past is to cut off inflorescences when the flowers are in bud and bring them back into the lab until they open and can be taken out for single visit experiments.  You need to have a set of control inflorescences to make sure that moving them doesn't result in pollen deposition on stigmas of course.

By the way do you mean "effectiveness" or "efficiency"?  They are not the same thing.

I agree with Jeff Ollerton, do you mean "effectiveness" or "efficiency"? 

Is also necessary register the different pollinators and its number of visits (of each different specie).

You need describe best your case.

I also do agree with Ollerton and Ortega.

the another  possible method is just collect the flowers immediately after the single visit of each/respective pollinator. Similarly collect flowers after pollination (before withering)  and fixed them on cotton soaked in FPA solution. Dissect the stigma on a clean slide and count the number of total pollen grains (species under study).  These exercises would give you an estimate of pollinator effectiveness and pollination efficiency. 

maybe this paper will help you. good luck~

http://www.jstor.org/stable/2404649?origin=crossref&seq=1#page_scan_tab_contents

 If you want to determine the efficiency of a pollinator by looking at pollen deposition on the stigma, you still have to protect your flowers from pollination before you start your observations. You could shield the whole plant by putting a cage over it, in case you are talking about a small herbaceous plant. For a shrub or tree, you could use a cage around a branch, which is not hanging, but supported from the ground.

We've tried just  about everything in order to pollinate mango flowers without contamination.  The only way we achieved limited succes was by caging the inflorescence and maintaining  the pollinator (in this case common houseflies) enclosed and in constant contact with the panicle for a minimum period of ten days..

However it is very difficult to assure complete success as we had problems with non flying invaders such as ants and thrips.

In the case of hand pollination in vitro we managed significantly successful pollination as follows:

1. Cut panicles at anthesis of majority of flowers (>80%) within the lower1/3 of the panicle.

2. Bag panicles in a thermos at 13°C , rush to the Lab. and place in a nutrient solution or distilled H2O

3. Emasculate receptor flowers and keep in an adequate agar preparation

4. Exrtract pollen from donnor stamens 

5. Apply pollen to receptor flowers

Nevertheless I feel that it is almost impossible to acquire uncontaminated receptor flowers wihout some sort of bagging.

HINT. In mango timing is of the essence as hermaphroditic flowers are only receptive  at precise temperature and RH which in our case (Southern Mexico) occurs from 8 to 10 A.M.

I dont know what species you are working with or under what ecoclimactic conditions but may I suggest you try working along this line. You might get away with collecting clean uncontaminated flowers before undesired pollinators beat you to them.

Best of luck.

M.R. Gehrke-Velez