Overwatering is reportedly the biggest killer of houseplants. When a plant in a pot is overwatered, the excess water remains in the pot--completely saturating the plant's roots. Roots require air to thrive and survive. If their soil is filled with water, the tiny root hairs are unable to find any air in the soil and they drown. If the root hairs, which collect water and nutrients for the plant, are compromised--the entire plant is in jeopardy.
Whether you are overwatering your houseplants or your garden, the symptoms are the same. Plants often display similar symptoms to being underwatered when they are overwatered. Your plant's leaves will turn yellow and droop, sometimes dropping off of your plant. The plant's growth will be stunted. If overwatering continues, your plant will develop brownish and black fungal spots and perhaps a grey, fuzzy mold. The soil your plant is in may even begin to smell as the roots rot.
If you see water standing in the dirt around your plant for awhile after you water it, you probably overwatered. Do not just water on a schedule, make sure your plant actually needs water. Check the dirt around your plant before giving it water. The dirt should be dry on the surface. If your plant is in a pot, the top third of the dirt should be dry before you re-water it. Similarly, different species of plants have different water needs. Make sure you are familiar with the watering needs of your specific plants.