One cannot see their birds drink water regularly...I'm sure none or very few people can actually claim to having a sight of their birds sipping water regularly. The water bowl may look clean for the entire day but one should change it at least twice a day. I offer fresh water thrice a day to my birds. At night before switching off the lights, I personally hold the water bowl towards their beak and each one of the five birds eagerly drink water from the bowl. The male ones sometimes even bath in it while I hold the bowl.
Aluz has given great tip about offering them fruits and veggies which are rich in water content. one such example is spinach. You don't need to worry for your bird as long it looks healthy. You could even spray some water on it during day time which it will surely suck from the cage bars.