CAUTION!! HAND LOSS!!
There are two tricks that take a little common sense when working with a running engine. You might do damage to both body and car, if not careful!
One trick to find out if the belt is making the noise and not another part, (i.e. tensioner, alt. bearings, A/C pump... etc.) is to take a bar of soap and touch the outside of the belt area. Be safe enough, so as not to get a hand or part of clothes (long sleeve) caught around any pulleys. If the noise goes away, it's the belt.
The same scenario stated above, but this time, use Gasgacinch.
If you have something like 3M Tack spray, this would also work. Just be careful about the vapors and fire!!
If the noise is still there, then rule out the belt. Next thing is to remove the belt and spin the pulleys. One should make an obvious noise that you are hearing. Don't assume (it's the tensioner)!