Dry grass responds by wilting and rolling or folding the leaves. If the soil is very dry, it will be hard to insert a screwdriver into the lawn. If any of these techniques determine your lawn is dry, apply about an inch of water. If runoff is a problem, apply half and let it soak in. Then apply more.
Early morning is the best time to water.
Water only as often as necessary. Applying a little bit of water daily can be harmful since it can encourage shallow roots which make the grass less drought-tolerant. Stretch the interval between waterings as long as possible to encourage development of deep, extensive roots. Flip-flopping between the two can weaken your lawn.