What would cause the belt to walk on a tensioner pulley?

 

Pulleys out of parallel, an idler or other accessory with a worn sloppy bearing, a bent bracket/bolt on an idler or another belt driven accessory.

 

How do I use a straight edge to check the pulleys?

 

With the engine off! Use a string if you can't fit a straight edge. Hold one end of string on pulley A, span across pulley B, pull tight and hold other end on pulley C. If string hits pulley B before C, or if there's a big gap between pulley B, something is out of alignment. Then keep working your way up the alphabet!