1. Explore the Soil
The simplest and most effective way of knowing when to water is to explore the soil. Dig down and feel, smell and see whether the soil is moist.
2. Inspect your Plants
Another way to tell whether or not to water is to look at your plants. When plants need watering the leaves wilt, lose colour, and turn grey. If the leaves are firm, then the plant is fine. It's important to learn which are good indicator plants in the garden. No textbook can tell you, it's one of those acquired skills that gardeners develop.