My ferrets will go down any hole i present them with, they are curious/do what i point them to. But yeah, they will sniff around outside the hole and casually wander down, then just come out seconds after if empty. If occupied, they fly down. Just keen on finding a way of reading their sense of smell (hearing maybe) before letting go of them. Best i can think of, when using my older jill, is letting go of her with one hand, judging how fast she bolts into the hole, then catching her with the other hand.
Keen to see if theres a dogless way of marking. The ferrets know if there are rabbits