Before this gas tax refund bs, the local tribal station had a sign on the pumps for tribal members to to swipe their tribal ID first. Kinda like the Safeway club card thing when you used to get .03 off. So tracking tribal and non-tribal purchases is pretty easy.
I no longer go there since the State began it's refunding of all their fuel so I don't know if this still hold true. I may go digging tonight or tomorrow and perhaps if I remember I'll drive through and see.