These are so easy and cheap to rebuild that the only real way of checking wear is getting close and personal with the various metal to metal contact points, the appearance of the blueing and most of all the crispness and shine of the action. I have two one of which has had 15+ years of clay and rough/pigeon shooting but closes with the sound of a brand new merc door, the top lever is so far to the right any more and it would fall off :lol: cos it was replaced a few years ago, the stock too is brand new because it was almost never used. The newer one closes with less panache and the lever was central when I bought it 2nd hand, the owner assuring me it was sub 10k usage. Less than £200 would make this gun feel brand new and look a year old so basically really difficult to appraise off pics.
Basically it's a case of close scrutiny and trusting the sellers word regarding use. The good news is they're a fraction of the price you'd pay for guns that don't do anything any better and any minor problems can be fixed cheaply.