When I was a kid I used to worry that my doctors or RTs or nurses would know I was over using my rescue inhaler by some lab test or x-ray or something.  Yet the truth is there is no such test.  The only way they can tell if you abuse your inhaler is if you tell them, or if your doctor monitors your prescription drug use.  Of course, a wise doctor WILL monitor your prescription drug usage.

That said, if you are using your rescue inhaler too often, or a lot, that may be a sign that you do not have good control of your asthma.  Thanks to modern wisdom and medicine, asthma can be prevented.  If you find a good asthma doctor, work with him on finding a good medicine regime, and you are compliant with your medicines (take them as prescribed), you shouldn't need your rescue inhaler.  In this way, asthma can be controlled and prevented.  I bet this is what your doctor was referring to. 

So, if you want to learn more about how you can gain control of your asthma, check out this post. 

It's also important you be a perfect asthmatic.  Check out this link on how to become a better asthmatic. 

Good luck

Rick