Gamete expression while wearing glasses can often get you done to 3" is the fish are in good condition.
Below has been more of an academic interest except when working with smaller sunfish needed to breed under controlled conditions in a pinch. I have yet to try it with RES, but I can sex a lot of sunfish by isolating individuals in a 10-gallon aquarium for a few days with good lighting and temperature then introducing water from a known female sunfish that is getting ripe. Usually the suspected males will start digging within a day. Introduction of a ripe female of similar size to the male can later stimulate a dance that females do not do.
Aquaculture Cooperative Research / Extension Lincoln University of Missouri
