Is there any seepage or dripping under the water pump? You can try this: Release the tension on the belt by using a 17mm socket with a long handled wrench. Put it on the belt tensioner and move it counter clockwise.
There is a hole in the tensioner, put an allen wrench or something in the hole to keep the belt slacked. Grab the water pump pulley and turn it. listen for any grinding noises. Then shake it side to side it shouldn't move, if it wobbles it's shot.
Spin the other pulleys, except for the crankshaft of course. Listen for any noises. If you haven't changed the idler pulley recently, I would change it for the heck of it. Check the belt to see if it's worn or shiny, like it would slip.
I've posted a picture listing the pulleys below. Good luck!