Look for little black pinhead sized things crawling around the scales. You'll find them especially chillin up near the nose and mouth. Also, you can find them in the waterbowl if the bp soaks (little black flecks).

There really isn't a way to prevent them from getting them that I know of. Just keep a clean enclosure quarantine new snakes. But, once a snake gets them and you get rid of them, they don't normally get them again unless introduced via a new snake or something similar.

Most members here swear by a product called PAM (Provent A Mite). I've never used it, but I've been told it works wonders. A lot of herpers have a can just in case.
linky