2000 Oldsmobile Bravada serpentine belt tensioner

I've got a squeling coming from the front end of my vehicle. I'm pretty sure it's the tensioner, but would like to know if there's a sure fire way of checking iit out without removal. I took the belt off, and rotated each component. They all turned without any squel or catch. When I put belt back on, and turned motor on it still squelled, but tensioner kept slightly bouncing up and down as squel pitch seemed to keep pitch with the bounce.