While most people are aware that not watering a plant will cause it to die, gardening novices may not be aware that over watering can kill a plant as well. Unfortunately, this lack of awareness can lead those ignorant of the issue to inadvertently make the problem worse as they mistake the signs over watering for under watering.
You won't notice root rot unless you specifically set out to discover it, since root rot will occur under the cover of your soil. Unhealthy roots will be mushy and will have a brownish color. In addition, in some cases of root rot, the roots will give off an unpleasant smell. If you suspect that you may be watering too often, you may want to check the roots for problems.
Leaves can fall in a variety of situations in an unhealthy plant and for various reasons besides over watering. One of the ways that you might be able to tell if the problem is from over watering is if the leaves that are falling are either yellow, or if the leaves that are falling are both young and older leaves.
This is a fairly obvious sign of a problem with over watering in a potted plant. If there is standing water left over after your watering session, you are very likely watering either too much, or too often. Reduce the frequency of your watering, or use less water. It is natural that water appears in the liner directly after watering. The problem here is if some time later, the water remains.
If mold begins to grow anywhere on or around your plant, this is a sign of over watering. This includes the soil surrounding your plant. Mold will likely appear as a soft, whitish patch. Mold grows when there is excess moisture.