Be assured this is not a strictly academic concern so I am hardcore when it comes to sexing BG because messing up gets expensive.
Focusing on Northern BG;
Dorsal spot is not a reliable indicator as is has more to deal with mental state that is generally independent of reproduction.
Urogenital opening works when females mature. Opening I am concerned with is the most posterior as I am not not able to sex a fish based on appearance of the poop. Expressing gametes is better. The yellow coloration of male's vent area works best on the first type of male below.
Northern BG males come in three flavors with different names according to who you are talking to. First is bull / primary / nest building that has the large opercular tab, dark spots on nape and sometimes dorsal part of flank as well as a generally blackish cast ventrally. Such males can possess solid bars on flanks. This variant most people will pick up off nest or out of raceway and call a male with little hesitation. Second differs first by having a smaller opercular tab and less developed black spots. Solid bars can also be present. This variant can be confused from a distance, especially when keying in on opercular tab and sooty look of the first type. This dude looks like a lady and seldom makes a nest and when he does has a hard time sticking with it. The third type is small like a juvenile without taking on pattern of either female or male. These guys sneak into nest and drop a load when female is extruded eggs with the first type of male doing most of the work.
Northern BG females have a freckling pattern on flanks, generally smaller opercular tabs that can occasionally be larger than those of the second male type, and generally a yellowish belly early in breeding season. As breeding season progresses, actively spawning females start to look silvery as pigments are depleted as they are invested in eggs.
