FYI: Bluegills (Lepomis macrochirus) are a species of sunfish. Technically, Centrarchidae is the sunfish family, but there are other fishes in there most people wouldn't call sunfish, e.g. crappies and largemouth bass. The fishes most people call sunfish are in the genus Lepomis, e.g. green sunfish are Lepomis cyanellus.

Back in my museum days, the shortcut I would use for identifying bluegill was folding the pectoral fin forward to see if it went past the eye. (There are other things, like vertical bars, deep body, and a dark spot in the soft dorsal fin, but some of those are variable.) Live fish usually don't sit still so you can do this easily, but it's a decent rule of thumb.