Bonsai trees require constant watering as the porous soils used cannot maintain moisture for long. One way to tell if your Bonsai needs watering is to simply place a wooden chopstick the in soil 2 to 3 inches deep for ten minutes. If when pulled out the chopstick is damp, no need to water. If the chopstick is dry when pulled out then it is time to water your Bonsai. Another way is to simply stick your finger in the soil and gauge the moisture level. Finally you can use a moisture meter to ensure accuracy.
If your Bonsai is in a training or growing pot, give it a good soaking. Using a gentle flow watering jug so as not to disrupt the soil or top. Keep watering until draining starts. If your growing/training pot can fit into a larger container, water the Bonsai the same way recommended below.
Find a large container that can fit your Bonsai pot fully. Fill in this container with collected rain water, spring water or filtered water. Until it reaches just under the lip of the Bonsai pot. Submerge the Bonsai pot into the container making sure the tree is above the water line. Leave in this state for 10-20 minutes. Do not leave for too long to ensure the roots do not rot.
Take out the Bonsai pot and let it drain well.
Additional Watering Notes
If you are not able to water the Bonsai in a container, or you are using pellet fertilizers, then use a gentle flow watering can. Water until the water starts draining. Wait a minute and then repeat the watering. Repeat this cycle for at least three times.
During the summer, if your Bonsai is outside, you might need to water more than once a day on very hot days. Early morning watering is ideal. Evening or nightly spray or misting is also very helpful.
Make sure to get to know your Bonsai and how much water they like. Once settled in, you should be able to tell by the leaves/needles of your Bonsai whether it is happy or needs a drink. Droopy leaves or needles, lack of shine and sagging foliage are a sign your Bonsai needs watering.
Although be careful not to overwater, to much water can cause root rot, which is a much harder issue to deal with than under-watering.