Sight down all the pulleys to see on which one the belt is off to one side a little, then look in particular at the pulley that comes just before that one. If the tensioner is the suspect, look behind it to see if there's some corrosion built up that is preventing the mounting plate from sitting squarely. The bearings in the idler pulley can develop play too. That will typically cause the outer edge of the belt to fray.