Eyes- need to be clear and alert. Any cloudiness can indicate eye caps or BAD health
Mouth- any signs of mucus, bubbles, and (worse) cottage cheese like substance- not good. Also look at the "lips"- if the mouth doesn't form a nice seal, there is normally something wrong. Ask them to take a picture of the mouth pulling down so you can see the teeth. (see next point)
Gums- gums should be a nice, clean, and clear white color. No red, brown, or yellow spots.
Cloaca- I like to have a shot of this area because mites can hide up there even if the rest doesn't appear to have any on the snake. also, be on the look out for blood (anything abnormal).
Lower Intestine/Upper Tail area- great indication of malnurishment. If the area is kind of concave and smaller than the rest of the body, the snake is underweight.
Belly- Look for scale rot signs here. Easiest to spot on the belly area and they get burned here if the tank isn't set up correctly.
Backbone- another good sign if the snake is underweight- you shouldn't be able to really see the back bone of the animal. If you can, it's malnurished.
Also, while looking for all these things, look for signs of dehydration, stuck shed, mites, wounds, etc. General stuff that can be easily corrected.
Whatever you do- MAKE SURE YOU QUARANTINE THE ANIMAL! Trust me on this - I learned the hard way, took in some rescues and BAM! Whole collection got hit w/ RI. Don't take the chance.