It comes from intense training at the illustrious School of Spotty/Swirly Cories.
I just look through the Cat-eLog a lot and am on the forums all the time, reading what people have and looking at pictures. Some fish, like you said, are quite common, so when you see them often, you get to know what they look like. *L* If you look through the Cat-eLog at the cories you can see what a huge array of designs they have!
My C. Leucomelas, Freddie, is the fish on the far left in my signature. They are very cute fish!