Belts will loose after putting them in when they are NEW sometimes they need adjustment. Check for looseness. If you can put your foot on the break put it in drive and increase the throttle you may be able to ID the one giving you trouble if you have some one help you. If you can do it then take some brake clean or some windex spray it on each belt until the bad one goes quiet.
"Careful where you Stand"

You could take off each belt except for the power steering and drive it a short way with out causing any problems to isolate which belt is giving you trouble. Also you can hand spin each pulley to feel for drag also check the idler pull for the a/c as these can get rough.