How can I tell if my birds are thirsty?

[enggass]

I have 2 new waterers and have stopped filling their old one. I do not know know for sure if they are using the new ones much. They are large and it is hard to monitor the water level - I see them drinking at times, but do not have the time to sit around and watch for any length of time. What behavior would they exhibit if they were thirsty?
Steve

 

[33yardbirds]

I give them fresh water every morming. Sometime they drink alot other days they don't. They'll drink when they're thirsty.