Dogster.com has a good posting about the subject.
For Wiggins and Renaldo I keep two bowls of water out at all times. One in the kitchen and one in the bedroom. The dishes are always in the same spot. I change the water daily and sometimes twice, depending on whether they've been in it a lot. When I do change it I always fill the bowls to a certain line, so I can monitor their intake. Of course they have to have ceramic bowls!
As the article at Dogster notes, increased or decreased water intake usually means something.
In summer months, and lately since NYC has been hot as hell, I notice they are drinking slightly more.