Thanks for the kind words Seth. Choosing tree frogs is tricky because many (particularly those that are freshly wild-caught) are not active during the day, when most of us visit the pet shop.
My first advice would be to avoid wild-caught if you possibly can. Secondly, ask the staff to feed the frogs in front of you - an active, eager frog is usually a healthy one and when we give advice to people on the forum who are worried about their frogs, one of the first questions we ask is if the frog is eating regularly/well and lately.
Apart from that, make sure the frog has no injuries and keep an eye out for any strange patches on the skin or redness (particularly on the limbs and belly, although on many tree frogs with white/pale bellies, a slight pinkish tint is normal). Patches are usually an indication of sickness or injury.
Lastly, try to avoid overly skinny frogs, particularly if they don't react well when fed - they are likely suffering from stress and/or a significant parasite load. Keep in mind that male frogs tend to be skinnier than females, particularly in the case of tree frogs.