There are several ways to tell if a cactus plant needs water. When the soil in the pot is wet it holds together, when it’s dry it becomes powdery and falls apart. You can try sticking your finger into the cactus plant pot as far down as you can to test for moisture. You can also stick your finger into the drip holes on the bottom of the pot to check for the moisture around your cactus plant. The easiest way is to simply look at your cactus plant. If the cactus plant has become soft, supple, or even a little shriveled; it is definitely time to water.