How do you know if your asthma medcine is not working?

Couple of things: If it is not relieving your symptoms or if you are having trouble breathing you need to see your doctor.

By how you feel: If you still feel short of breath/coughing/wheezy/tired, your asthma medicine may not be working. It may be that the canister is empty (don't hear anything on shaking; floats in water); old/expired, or you may need stronger medicine.