Honestly... nothing. Now that you've bought it, it doesn't really do much good to ask what you should look for, does it? I mean, really... people should ask these questions prior to purchasing something, don't you think?
As far as safety goes, cylinder lock up is probably the best thing you've already checked, but take a peek down the barrel (unloaded of course) and check for any bulges or signs of a stuck round or squib load.
Always ask first, then buy. You might someday prevent yourself from buying a lemon.