Apart from the general level of finish....metal to metal ,or to wood,,,,
perhaps attention to detail where one normally might not look, ( though of course we would smile ), like the short rib area under the fore end, that can be pretty crude on a cheap Gun....also a lovely pair of Locks is a good sign.
I guess I dont really have a check list, just kinda look at all of it n count any Ugly bits against the nice bits
cheers
framc