"I don't breath in through my nebulizer." You must mean your Aerobika, right?

I could tell Aerobikas weren't working when the valves would collapse and there's no resistance. It's easy to tell-different sound and feel. I'd think the filters could become plugged making it harder, too.
I've put a couple devices in a ziplock and labeled "old" when they still seemed ok, but it was time to change.