Water is critical to keeping plants alive. The right amount of water means your plants will thrive. Overwatering can kill a plant. The roots pick up oxygen from the soil. If the soil is water-logged, the plant can't get the oxygen it requires and it will die. Hot, dry weather means the plants will require more water. Windy weather dries plants out as well since water evaporates from the leaves of the plant.
Look for changes in the color and vibrancy of the leaves. Know what your plants look like when they're healthy and well-maintained. It's easier to tell when they're water stressed. Grass, for instance, will go from a bright green to a dull grey-green when it needs water. Yellowish leaves on trees can mean they're not getting enough water.
Keep an eye out for drooping leaves. That's the stage right before the leaves wilt. Some plants like zinnias and tomatoes, for example, can look severely wilted and pop right back after a good long drink. Others, like bacoba, don't recover from being wilted. Corn will recover from being wilted, but its growth is stunted. Watering before the leaves wilt is better for the plant.
Test the soil. Insert a wooden skewer into the ground. If it's difficult to push in, the odds are the ground is dry. Dig into the ground with a hand-shovel. If the soil is dry at a depth of 3 inches, it's time to water.
Measure the rainfall. If it's less than an inch per week, supplement with extra water.
Look in the pot of a potted plant. If the soil is dry to the touch when you stick your finger in, or if the soil has pulled away from the sides of the pot, it's time to water.
Pick up the pot. If it feels light, it's time to water.