The biggest difference I can see is without taking them out of the water. Northern BG have no white or pink edging to their fins, while CNBG have this edging, especially on their tail fins. The edging shows up at a pretty small size...
I have seen blueish white edge fin tinting on northern BG at my dad's old pond. Some of the small 2" BG had very light sky blue tinted fins that got lighter at the edges, others appeared to have very transparent fins. With extremely clear water and a dark bottom it was very noticeable watching them side by side.
