The general advice is to look for clear open eyes, watch him for a long time and see if he closes them for periods of time. Also be observant of how active he is.
Hold him and get him to open his mouth. Check for signs of any bacterial infection and make sure the inside of his mouth looks pink and healthy.
Check claws and limbs for injuries, infection, signs of MBD.
Ask if you can take a fecal to your vet before you purchase or, better yet, if you can take him plus the fecal for a wellness check.
I hate to put a general ban on pet store purchasing....if they seem to be doing a good job with him maybe they're okay.
I would quiz them with some basic husbandry questions and see how knowledgeable they are.
If someone sells you a car they expect that you may take it to a mechanic, the same rules should apply here. You may have to leave some collateral ie: credit card but if they don't let you have him checked out I would think again about purchasing.
-Brad