99% of the time its the tensioner pulley, the one your gonna move to get the belt off and the one to its right, the idler.
when the belts off check it by bending it over your finger grooves up so it will show any cracks, if cracked and worn the belt could be the problem.
another way but its far from safe really, it to reach in and touch the bolt that holds the idler and tensioner when its at idle, it will feel like its vibrating if its bad, or grinding vs the other that should be smooth feeling.
and a bouncing tensioner is another one that can squeal, start it up watch it for movement, they move, but it should not be bouncing a lot. some times if the tensioner pulley is bad your stuck buying the whole tensioner with a new pulley. depends on the car tho.
