Generally you can tell a good healthy fish in several ways. Healthy fish generally have good color, erect fins, shiny scales. Its easier to explain things to avoid than things to look for actually. Avoid any fish that has deformities like a bent spine, missing fins or bent fins. Look out for fish that can not hold themselves horizontal in the water and seem to tip either up towards the surface or down to the gravel. Watch for discoloration on the scales, white patchs of fluff or any white spots that grow out of the surface of the fish. These are all signs of a sick fish.
I think bloozoo2 mentioned this which i agree with, as i've always done it, get to no your fish, it takes a while then you will no, it's like when you have children all the time you put in, you no when they are unwell.
I think if a fish is exhibiting it's behaviour as close to as it would in the wild then that would be a great example of a healthy fish. Also, I'd say that in most fish breeding is an example of healthy fish.