How do you tell if you have poor drainage? First, your plants won't look happy. (Surprise!) The foliage will look dull and lack the luster and intense color of a healthy plant. If it is a blooming plant, it may produce few blooms or none at all. When the condition becomes severe, the plant will drop its leaves from the interior first, eventually working its way to the leaf tips.
The second sure sign is if you are not watering much but the ground stays continually wet or even has moss or algae growing on it. The soil may also have an odor to it. Every time plants are watered, the soil temperature is lowered by up to twenty degrees. Most plants are stimulated to grow as the soil temperature warms up. If the soil is always wet, the soil temperature will be cooler than the plant desires and it won't grow much.
Poorly draining soil also attracts bad bacteria that can attack the root system, in addition to providing less oxygen for the plant. If you think you have bad drainage, gently lift the plant out of the ground with a shovel--being careful not to damage roots.
If the soil is wet at the bottom of the hole, dig it deeper and back-fill with at least six inches of gravel. Then build a mound that will raise the plant 3-6 inches higher than the surrounding soil level and re-plant so that the top of the root ball is level with the top of the mound. If that doesn't work, you may need to find a different location for the plant.