I don't know how to test a can of bear spray without using too much, but last year after packing my trusty can of spray with me for about 4 years I tried to use it on a domestic cat that had been beating up on my cats, alls that happened was a small dribble came out and ran all over my hand. I don't know how long it had been a dud' might have been that way from brand new but I do know that it had given me a very false sense of security for some time. Actualy within about 5 minutes of getting the spray on my hand I managed to wipe my face with it and found that really unpleasant, so I guess if I had been involved in a bear attact I coulda rubbed some in his eyes as I was going down lol. Really I shiver to think of the amount of time I spent in the bush alone mushroom picking actualy thinking I had some sort of defence with me in case of an attact. Now I will buy a new can every second year at least, I can just imagine what it would have been like facing off with a bear or cougar and having that damn can misfuntion like that. I also always pack a hunting knife with me but I really think all that would give me is a faint hope of a chance at best. I talked to a guy last year that had been mauled by a black bear and he said it was just like being in the middle of a dog fight instantly with all the snapping and clawing being completely overwhelming and the bears arm strength being far greater than any humans. BTW no cat was actually hurt during this as I was the only one that got to "enjoy" the spray it was actually a source of great humour for some time around my place to everyone except me. What a lucky guy that was to kill that bear with a stick.