Avoid snakes that have sores in their mouth. After checking the mouth, slide your fingers along the spine to check for any lumps or bumps. Look very closely at its body for any ticks or mites. Ticks can look like a scale when they're small. If you see one, there are probably more.
Mites can be even harder to see.
Mites will gather around the eyes and mouth, or that may just be the easiest place to see them. They look like tiny bugs or little specks moving around on the snake. A snake with mites should be treated before the infestation worsens. Check the anal scale to be sure it lies flat against the body and there are no visible signs of crusty matter or smears of diarrhea. And you don't want to start off with a snake that aggressively and repeatedly strikes at you whenever you get near its cage.