Unlike a houseplant, bonsai trees use a "free draining" type of soil because their roots cannot tolerate "wet feet". In addition, they are grown in significantly less soil and, therefore require more watering.
Factors such as tree location, temperature, lighting conditions, quantity of soil used, and the changing seasons will determine the frequency of watering. You can get to know when your tree needs to be watered by observing the foliage, testing the soil with your index finger just below the surface, or just by the weight of the pot. (The drier the tree, the lighter it will feel.)
To take the guesswork out of watering, we recommend an inexpensive moisture meter which works very much like a thermometer. Insert it into the soil and the movement of the needle will tell you if it is time to water. You can also try one of the low tech approaches such as the chopstick or wick methods.
Take a chopstick that has no coating on it and stick it in the soil of the pot near the edge. After 15-20 minutes take the chopstick out of the soil and see if it feels damp. If it is damp you don’t need to water.
The next time you repot your bonsai take a strand from a string mop and thread it through both drain holes in the pot so that the string hangs out each side. You can tell if it is wet inside the pot if the string still is.