"Bear Spray" is basically repackaged pepper spray, at least as far as I know. So I would think that one would look the way one would for OC- Shofeld Heat Units. This measures the hotness of the pepery-ness.

Hard core OC is supposed to run someplace in the realm of a million SHU. What you will want to really be concerned about is range. And remember, a sidewise breeze means all you are doing is getting it on your buddy. A wind in your face....

Oh, and carry some bells. Saves you the hassle of finding out if the bear spray is any good if the bear gives you as wide a berth as you should give him/her.