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
I have several hundred photos of RES all from my pond, taken from early spring in February to late fall in November located on two different Photobucket accounts. I still haven't figured out how to catch them through the ice yet. Those pictures show all sizes. I have a lot more photos than that taken throughout the year on my computer.
The urogenital difference is there year round with just small percentage of RES that fall in that in between area that might be questionable. With the ear tab coloration and size there is a bit more overlap between male and female RES. Water clarity and daylight conditions do affect overall coloration so there can be a lot more variance in that area.
I am pretty sure this is non-breeding male (7") RES caught today. Keep in mind my water is pretty green and water clarity isn't real great right now so my fish look a little washed out. After looking at this one again this could be one of those in-betweeners I occasionally see.
More often than not you can express gametes in the fall once the fish are back into good weight. Volume will be low and care must be taken not to cause damage. A great deal of feces will obscure view. You will likely note eggs expressed will appear smaller and likely "greener" than you see in spring prior to start of spawning season.
Aquaculture Cooperative Research / Extension Lincoln University of Missouri
Nothing with a gentle squeeze on this 8" RES, I am very confident this is a male though. We have had almost six inches of rain in the last week so the pond is high and muddy, visibility is currently 4" and water temp is right around 65 degrees. Colors are very washed out on this RES. Note that the ear tab coloration is slightly larger on one side compared to the other.