The best way to check any SKS up-close would be to disassemble it and make sure the parts function. They are pretty easy to take down and put back together. I don't know if the shop owner would let you do that though.
You can find instructions for that here:
http://www.surplusrifle.com/sks/disassemble/index.asp
Try this next time you go in at either model SKS (Russian, Yugoslavian, etc):
Pull the bolt carrier all the way back (just grab the handle) and it should lock. Take a look at the face of the bolt and see if there is any gunk/pitting. If there is a little bit of grime, try to see if it will easily wipe off.
Bring a small flashlight that you can shine through the chamber and look through the muzzle. See what kind of condition the rifling and grooves are in (if they look worn), if the bore (the innards of the barrel) is shiney or dark/worn looking from excessive use or lack of cleaning, or if you can see some pitting in there.
Ask the shop owner if he (or you) can take off the handguard that has the gas cylinder inside of it to check out both the cylinder and piston for pitting or corrosion.
As a last bit, ask the shop owner if you can stick the bullet-end of a 7.62x39 cartridge into the muzzle to check how tight the rifling it is. Typically the tighter the fit the more accurate it will be.
I own a Russian and Yugoslavian M59 (the one that never had the grenade launcher) SKS. The Russian SKS has a chrome line barrel which makes cleaning a cinch, and the M59 is just as nice as the Russian one, but has a steel barrel. They are both decent shooters and you can't go wrong with either.