Mites are tiny moving specks on the snake's body...& they hide under scales, places like their chin, eyes & nostrils too. They're red/brown/or black, about the size of a pin
point, so yes, take a white damp paper towel & see if any rub off...also keep him on white paper towels as substrate for a while, & keep looking in water bowl. Try using a
magnifier if your near vision is less than perfect.
I sure hope he didn't bring any home with him, but you'll have to look close & keep looking...their numbers can explode exponentially & kill a snake if you aren't vigilant;
they can also carry diseases so they're best avoided. Was your snake boarded where there were other snakes?