Grab yourself a little flashlight to peek down the barrel. Pitting, erosion, filthy barrel, obstructions, etc. should be relatively obvious.
Check the exterior of the rifle for signs of serious abuse. i.e. a cracked stock, rust and pitting on the exterior metal, large divots in the wood, bent metal, missing or difficult to move pieces such as the safety, work the bolt.
Other than taking it to a gunsmith prior to buying the rifle it's a bit of a lottery. Buying used items will eventually turn into a learning experience no matter how careful you are.
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