Fish can be very difficult to diagnose and treat, and many of the symptoms we see can be applied broadly to indicate that it is simply not feeling well. Some illnesses, like Ich, are very obvious. If you are unlucky enough to ever see those white spots, you'll know, and the treatments are well-documented and simple to follow. Other things can be much sneakier, and difficult to pin down. Medications can put fish under a lot of stress, and some species will be more sensitive to certain chemicals than others, so it is important not to put anything into your tank without doing a bit of research first.
The best way for you to know the health of your fish is to watch them. Take the time every day to look over their eyes, skin, gills, breathing, coloration - and to get to know what normal behavior is to them. You'll know if something is off when you see them behaving in a way that is different from the norm, and learn to recognize a problem.
If you feel that your fish are unwell, and are unsure of how to diagnose/treat, put up a post in the diseases
area of our forum, and someone may be able to help out.