Well, of course there is no guarantee that the fish you see, you are interested in (and you are going to buy) is totally healthy. A Fish can appear as healthy as it could be but in fact it has invisible inner problems. There are many ways, many things you should seek out to determine if a fish is healthy.

And here's a list of some:

Firstly, look for physical characteristics of good health. A healthy fish should have bright body color, not pale or dull. However darker or patchy coloration may be an indicator of stress and not of bad health. Before you buy a fish, find out how it should look under normal conditions. No open sores, boils, ulcers, peeling scales or blemishes. No visible parasites, such as tiny white crystals that look like salt or tiny black nodules that look like "blackheads". There are also viruses, like the Lymphocystis (cauliflower-like growths on the edge of fins). Be sure to examine the fish before buying it.

Eyes should be clear, not cloudy or popping out of the sockets. Fins are erect, especially the top fin (dorsal). Buckled fins can mean trouble. No ragged, torn or ripped fins. All of them should be intact.

Scales are flat and smooth, stomach is well rounded, girth of the entire body is of normal size, not bloated or sunken. Visible fish waste should be dark in color, not pale.

You should also examine their behavior! Healthy fish should breathe normally, not guzzling for air or hanging around the top of the tank where its mouth kissing the top of the water. Compare the gill movement of a fish with other fish in the tank. Extremely rapid gill movement may be a sign of stress.

The fish should interact well with another fish, as they pass each other, moving out, etc. Healthy fish should swim in a horizontal motion (not with its head up or down with a few exceptions) throughout the aquarium---not just creeping around the corners or hiding all the time. Dont pick a fish that allows itself to be bullied or likes to bully others! Spend some time watching the fish you're interested in buying maybe a good advice. Sometimes you'll see things you didn't notice right away. Watch how it behaves for a while.

Only buy fish that eat well! If possible watch them being fed, as this is very important. You wont have a wonderfully beautiful fish that only lasts a day, right?

It would be wise too to check fish prices. Unusually cheap price should be questioned, as there are some dangerous yet awfully cheap methods to caught fish today, like using cyanide and drugs. Cheap prices dont have to mean poor quality.

And make sure that the fish be caught with two nets. Thats the best, since using one net will usually only result in a crazy chase around the tank, making the fish extremely stressed.