These belts don't stretch, the teeth just break. The way they are checked is on the "tooth" side. The outside really does not mean much. I've seen belts with 50 miles on them where the outside looks crappy because it was barely rubbing against a piece of the cover. If you want to check the belt "officially" you have to take it off and bend it against the back. Then look for breaks or cracks in the teeth.
That being said, the roll of the dice would be up to you. 150k and you're not sure when (or if) it's been done.........could be a big ouch if you're wrong and you wipe out your valves. My car, my keeper, I'm gonna do it. Your choice.

MB