Alternator Belt Squealing

[KD88]

I've had this problem for a while, starts as soon as revs rise from tickover.

I've changed the belt and recently the alternator as well. It's the same whether it has a little less tension or as tight as I can get it.

In fact the only time it stops is if it's too loose.

I'm therefore thinking that the pulleys are out of line, although they appear to be ok by sight.

Is there a way of checking this or has anyone has any other suggestions?