First of all, make sure it looks healthy and is active! It should move around and try to get away, not just let you (or the pet store guy) pick him up. He should have a nice even coloring with a nice round body (not too fat, not too thin). You should also make sure the enclosure they are being kept in is standard for that type of frog (for example, a white's should not be kept like a fire belly toad).

from Shanna

No. Quarantine is a must! Never introduce a new frog into an existing enclosure without a quarantine period. The new frog could have a disease, a virus or parasites that you can't see right away. If the new frog is healthy, then it may have only recently contracted an illness and still be so well that it doesn't show. If you put a new frog in with the one(s) you have already then they could become ill and die.

Waiting is the hardest thing to do when you want to put them together, But it's better to lose just one frog than both or all of your frogs. Quarantine should be a minimum of 30 days. 60 days is best as tree frogs can carry diseases for weeks or months without showing symptoms and can pass these on to the existing healthy frogs. Quarantine tanks may be any clean, reasonably sized receptacle with minimal features. Generally a 10-20 gallon tank with a hiding place, moist paper towel substrate (changed daily) and a water dish is sufficient. Obviously, it's better the more you have in there, but this is the bare minimum.

Yes!! Regardless of whether or not your frog is captive born or wild caught and eating well, fecal samples should be brought to your vet for examination during its quarantine period. (more information)