When buying a SKS for a shooter - as opposed to collecting - I look at the muzzle condition, stock condition (cracked), and try to strip it down to see the barrel, trigger group, gas tube/pistons, magazine internals, and bolt. What I'm looking for is obvious damage and rust on those areas, and I like to shake the bolt to see if the firing pin rattles. There are some that don't, but they were made in 1949/50 with a spring-loaded firing pin - one could've been added from the aftermarket. I seldom worry about the bayonet, and part of the fun of shooting a SKS is making sure it's sighted in.
If it's a Soviet or Eastern Bloc SKS made post 1950, it should have a chrome lined barrel precisely because the primers of the military (as opposed to commercial) ammo have corrosive primers. The chrome in the bore prevents the salts in the primer compound from rusting the bore or gas tube/piston.
There are some excellent stickies in the Red Rifle forum, I'd advise you to study them carefully as they contain a wealth of knowledge on the SKS.