Instead of your doctor just "assuming" your asthma is controlled, he can look at your asthma journal to get a true assessment of how well controlled your asthma has been since your last appointment.
You'll need to work with him to learn what your asthma triggers are. He may recommend allergy testing, and you'll have to be vigilant to what else triggers your asthma (such as cigarette smoke or strong perfume) and learn to avoid those things.
You'll need to work with him to create an asthma action plan, and together you can make adjustments based on your description of how well your asthma is controlled, and your asthma journal.
You'll need to work with him to determine the best treatment plan. If your asthma remains uncontrolled, he may recommend new meds to try, and so can you based on your own research.
Yet only through your efforts, by being observant, and by keeping up on your asthma journal, will your doctor get a true and reliable picture of how the current course of treatment is working.
So, you can see, it's important you and your doctor work together, as an asthma control team, to get your asthma under the best control possible.