Inspect their eyes as closely you can. Their eyes should be clear, not cloudy. Make sure that their fins aren't tethered or damaged. If possible, feed the fish before taking it home. If it actively eats, it's a sign that it's a healthy, happy fish.
You should probably say it, and loudly. If the person is talking to you online, type it out so there will be documented proof that you stated your age. You...