Not sure how you would do this while driving, but the easiest way I've seen to check for this exact thing is to wet the belt just a little bit while it's running. It'll spray back on you a bit of course, but it should quiet it down for enough time to decide whether it is the belt or not. I did this on a mercury Sable once that squealed through new belts, pulleys, tensioners, etc. Never could figure out why it wouldn't stop squealing. Once I wet the belt and that worked, I just left it alone til I traded it.