I've used the water spray trick for finding a bad pulley. I actually have a can of belt spray to use for this. One problem with the spray is that the tensioner pulley is fully capped on the 5.4 and is mounted onto a bracket, attached to the engine. You could put a stream of water onto it and it won't hit the bearing surface or shaft of the pulley.
Getting a garden hose on the pulleys to use a stethoscope or even a stethoscope would be difficult with my truck. There is not a whole lot of space in there.
A tensioner pulley bearing failure is easily diagnosed by removing the belt and rotating the pulley by hand.
