There are several ways to tell when your plants need water. One common method is to touch the soil with your finger and feel the moisture level; if the soil feels dry to the touch, it's likely time to water the plant. Another way is to observe the plant itself; wilted leaves or a drooping stem can be a sign of dehydration. Additionally, some plants may show other signs such as yellowing or browning of leaves, or a slowed growth rate. It's important to note that different plants have different watering needs, so it's important to research the specific requirements for each of your plants.