A trained eye probably could but I can't until the fish are several weeks old. Then the differences in the way they swim and school becomes more noticeable. I imagine a person could if he netted some and looked at them under a magnifying glass but I have never tried it.
Late afternoon I watched the male on the nest for a while. He is definitely locked into position now. If I start to get too close he will start to leave the nest but he does not want to and I don't want him to so stayed a safe distance away. If I had a whole colony of them I could see, would be more aggressive and try to get some close pictures, but want to see if I can get some reproduction in this small pond so don't want to push my luck.
There was another male with a bed right next to this male, but it was a small maybe 3.5" RES. He seemed kind of sporadic, like he wanted to nest but didn't exactly know what he was doing. He would run other fish away, but sometimes chase them a foot or more away the nest before returning and chasing something else away. The two RES seemed to tolerate each other but stay out of each others nests. The 5" RES guarding eggs would never leave the nest, but instantly nip at any fish that came too close. He would sit right over the top of the nest, then periodically lower himself a little and very obviously fan the eggs with his pectoral fins. He could move with lightning speed if an intruder came too close, making a circle within the nest so fast it just looked like a blur.
Interesting stuff.
Edit: Added a picture. If you squint real hard and look exactly in the center of the picture the RES is there. The other smaller fish resides just behind this fish. RES do not seem to be near as particular about their nests as BG from my observations. These two "nests" are actually depressions I made shortly after adding the rock lining visible in the picture. I had rubber boots on and stepped out in the water a little ways and my footprints left a couple depressions. The RES have groomed and adopted these two man made depressions.
