Take a bore light with you. It's easy, just shine the light in one end and check out the surface for pitting.
If the person you are buying from has shot the rifle with corrosive and hasn't cleaned it, it will be visible pretty quickly.
It also means the seller knows how to pull the bolt and cover from the receiver. If it is clean and lightly oiled, it should be fine. Same goes for the bolt. Check it for cleanliness. Any rust in the interior is not a good sign.
Taking off the bolt cover and removing the bolt is a good idea so that you can check out the leade to the rifling. That is the throat area of the chamber. If the throat has been burned away by the hot gasses created during firing several thousand rounds, it will be quickly visible. If it is worn, the edges of the rifling won't be sharp but rounded and maybe not start for a couple of inches after the chamber.
This would be my concern. The SKS is a very tough and robust little critter. It was made to be issued to illiterate soldiers that would learn to operate and take care of it by looking at pictures, when no instructors were available.
Darn fine little rifles. If they were being made new, they would be valued much more than they are now. As it is, if anything relatively serious goes wrong with one, it is cheaper and easier to purchase another. The Chinese models sell as low as $169 new plus shipping and taxes of course. Use that as your guide. The Chicom SKS are new old stock that was made for their military and never issued.