Easiest thing to do is have someone crank the wheel and watch the belt for slippage.
I used to fight this frequently in my 91. As you tighten the belt with the adjustment pulley, it reduces the amount of wrap on the PS pulley. So the belt gets tighter but there's now less contact on the PS pulley. Not a great design.
What I did was loosen the adjustment pulley all the way inboard and rotate the PS pump to tighten the belt. This will leave the pump at a slight angle, but makes no difference to PS operation.
