Usual car hunting stuff really; log book, service intervals, panel alignments etc. As for 172 specific, take it out and give it a burn, check for rattles in the cabin and knocking on full lock, other 172 bugbear is the 4k rattle which is the exhaust centre piece. Boot it in 4th past 4k and come off the gas, as the needle passes 3.9/4k on the way back you might hear a buzz that could get more noticable as the miles clock up.
What condition/price/mileage is the one youre off to look at?
Z