There are many different ways you can tell when your plants need water.
1) Weight of your container, get used to how heavy it feels when they are fully watered and get used to how they feel when your plants start to droop from lack of water.
2) Soil Tester, Yep they make these that test how damp your soil is.
3) Drooping plants, your plants wills start to droop when they need water. They will also droop when you overwater them so make sure you know the last time they got waterded.
4) Stick your finger in the dirt chances are if it doesn't feel moist there isn't much water left.
From my years of experience I find that vegging plants usually need water about 2 times a week (if that) and that vigorously growing/budding plants need water every 2-3 days and of course it depends on the size of container you are using if its a smaller container it will need water more frequent and the opposite for larger containers.