I have an Olive Tree about 2 foot tall in a smallish pot. The only way I could tell it needed watering was that the tiny olives it bears were a bit wrinkled. I gave it a good drink and the olives plumped up. They don't seem to droop like other trees so a drink every so often is probably best.

Although they are drought resistant and survive in very poor soils, they will appreciate regular watering. I would do the 'flood and leave' type of watering, giving them a whole can and then waiting until the compost is very dry before doing again. Be careful that the base of the pot doesn't remain wet and rot the roots, but if you have good drainage in the pot as you should, this shouldn't happen.