RLS pretty much covered it. Don't be afraid to ask the breeder questions, even the ones that seem stupid, if they're good, they'll have heard it all before and will be able to help you even after the purchase.
It's more common sense than anything else but like RLS said, tail nips (Although some beardies can be born with kinks in their tails) and/or missing toes.
Generally speaking buying from a experienced breeder you're more likely going to get a good baby that will have been looked after and fed well.
Good luck, you'll probably get more, they're infectious