Use a piece of heater hose like a stethoscope and listen to each pulley (BUT WATCH THE DAMMED FAN.) Or pop the belt and turn and wiggle each pulley looking for abnormalities.
BUT, belts generally don't whine. They chirp, squeak, or squeal. If you are hearing a true whine, it most likely will be the power steering pump or the alternator. And the P S pump will cange whines when you rock the wheel.
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