What I usually look for first is that they are clean, clean, clean. Especially their rumps. Any messiness, wetness or dirtiness could mean wet tail and that's a heartache, even though it can be handled these days. Best to avoid that. Also, look for shiney, alert little eyes. Don't be put off if the hammies scatter, they'll be a bit anxious if they aren't handled frequently. If they are being handled well, they should be curious enough to walk up and sniff your hand.
Most of all, take your time and don't let anyone pressure you into taking any hammy because they want a sale. Others on the site will have more info for you I'm sure. Good luck.
