The top of your soil may be dry while just underneath it is drenched. When you check your soil you should check down 1 -2 inches or at least 1/2 the depth of your pot.
The easiest way for a beginner to do this is simply by inserting a wooden chopstick into the soil one half of the way between the rim and the trunk. Pull the chop stick out to check, the dampness of the wood will tell you how moist and/or dry your soil is.
As you become more experienced other things will tell you when to water such as the color of the soil, the weight of the pot, the look of the foliage, etc.
With the proper soil mix you can not water too much, the excess will simply run out the bottom. I thoughly wet all areas of the soil surface until water flows freely out the drain holes, wait about 20 minutes and then do it again.
Watering is a difficult skill to master, you should read the links
here and Brent's excellent article
here.
Will Heath