Make sure the barrel is clean. Bring a bore light with you, they cost about $5.
Remove the bolt from the rifle. put the bore light at one end and look down the other. A bore that hasn't been abused or neglected should look shiny, and the rifling should look sharp, not rounded.

If the bore looks grayish, this is ok too- it just means the bore might be fouled or slightly pitted (or frosted). If the bore looks fuzzy, bumpy, black, rusty, or the rifling looks chipped or rounded, the barrel has been neglected or has shot alot of corrosive ammo without proper cleaning- buying one of these is a crap shoot- it might shoot fine but foul up quickly, or might not shoot well at all.