There are a few ways to test the moisture level in a plant:
• The first would be to use your finger (if you don’t mind getting dirt under your nails). This works best with smaller plants because you have to go about 1/3 of the way down into the soil.
• Test the weight of the plant. If you can lift the plant very easily, it is probably dry. Try lifting the same plant after watering it to judge the difference.
• Soil Probes are metal, or plastic rods with several notches in them which, when inserted deep into the soil, will pull out soil samples. You just feel the dirt to tell how wet or dry it is. Soil dries out unevenly in potted plants, so you should extract soil from different areas around the plant before deciding how much to water.
• Moisture Meters will tell you to what degree the moisture level is at. It has a metal probe which you insert deep into the soil. At the top of the probe is a scale which registers the moisture level. This is pretty accurate in telling you when it is time to water, except for certain soil types and high pH levels.
There is no set way to check the moisture level of plants. Some ways are better than others for different plant situations. Use a combination of methods to determine what works best for each plant.