power steering belt falls of when car starts

I have a 2000 dodge neon and last night while driving home the steering wheel locked up and became extremely hard to turn. This morning i looked at the car and the power steering belt came off so i put it back on and started the car up but it just came off again. the belt is about a year old and doesn't seem to be damaged in any way. i've been told it could be the alignment of the pulleys but they all look to be aligned and they're stable. So what could it be and how do i fix it?

Improper tension?

How would I test that?