One good way to tell CNBG or at least BG with a good percentage of CNBG from northern BG is when they are in the water. Northern BG when in the water have an iridescent yellowish/green tail(caudal) fin in particular but their dorsal and anal fins will be that coloration as well. On CNBG, they will have that coloration too but the fin will also have a very distinct edging of coloration to the fins. The smaller ones its not to vivid but in adult fish it is. This coloration is usually a white to red color. Also, CNBG usually show distinct bars on their sides when in the water. Northern BG have the bars, but they are usually only visible under certain circumstances and their tends to be more of them. Here are photos I took of CNBG while in the water.