You'd know if your snakes have mites as they will crawl on your hands when you handle your snakes. When I handle my snakes I make sure to given them a good body rub and check my hands thoroughly for bugs as some stages are very small and hard to see. Sometimes you can see them on the snake as they look like black bumps. Your snakes will spend lots of time in their water too,generally. They're nasty little buggars and you have to keep on top of them. I have used "nopest" strips and they seem to work but the effectiveness of the strip seems to dissipate quite quickly. I have an isolation tub that I put the snake in, with a couple of small pieces of nopest in ventilated pill bottles and put the big section of nopest in the enclosure (seal the enclosure as well as possible). Then I clean the enclosure thoroughly and repeat this in 9 or 10 days to kill off the next hatch of bugs.