You'll see little creepy crawly lack dots crawling on your snake, stuck around the eyes, in the heat pits, under scales, etc etc
If they're on the snake, consider them to be EVERYWHERE
You would then proceed to treat every single cage you have. I like to do preventative measures and spray periodically. I also spray around the bases of racks.
Check in between the scales of you snake and check the eye sockets.
My recommendation no matter what always have a can of PAM (Provent A Mite) available and treat any new incoming snake why in QT (no matter where or who it come from as a precaution).