There are two ways. One is "popping" where you kind of press on an area near the vent to make the hemipenes of a male show. You have to have someone experienced show you how to do it though, or you'd risk damaging the reproductive organs or something
There is also a method called probing. The person uses a tool called a probe and inserts it into the vent. It goes deeper in males than females I think. Again, you must have an experienced herper show you before attemptingi t on your own.