I think sexing LMB is much harder than Northern BG when there is no milt/eggs present.
When I'm sexing Northern BG, I go by the rule of thumb that if I'm not 100% sure it's a female then I consider it a male.
Same for a male. If I'm not 100% sure then all bets are off. I also agree with Jim, don't even start going down the hybrid or different species Bluegill path in regards to male/female.
Here on the forum I consider "BG" to be synonymous with Northern pure strain Bluegill.