Here are a couple ways to tell if it's time to water:
1.) Lift the Pot method - Before you water (when the soil dries out - See 2), lift the pot, notice how light it feels. Really get a feel for how heavy it is. Then water the plant. Pick up the pot again, get a good feel for it. It will feel noticably heavier. Everyday check the pot to see how heavy it feels. When it gets dry, you will notice it feels light again.
2.) Finger in the soil - Put you finger in the soil about an inch deep. If it feels bone dry, you need to water. If it's still moist, give it some more time before watering again.