Estimation of the amount of bacteria
This is a very standard method. The idea is that you have a culture of bacteria on an agar plate. During the incubation each bacteria forms a colony. After a few hours of growing the colonies grow and become visible for human eyes. So you can easily count them.

Then you can compare the number of colonies before and after washing your hands with the sanitizers and soaps and find which method has the lower amount of colonies on average and therefore whihc one is the most efficient one.