This is one of the things I agree with Crowley on. Everyone is a magician. Everyone focuses their will on things, sends energy to it and creates their own reality. Remember, where awareness goes, energy flows. Just most people do a terrible job at it. They don't control their thoughts, emotions and actions so their energy is chaotic, and their results unpredictable.
Now, if you are asking how to know if someone works at doing ritual work and proper directed focus of intent, the easiest way to check is by examing the aura. A well defined aura, typically with light colors, is a good indicator. Of course, that is not an infallible method as the aura reflects the current mental and emotional state. Personally, I look at a person's life. Do they truely understand how to live without manifesting trouble in their life? Do they live in balance? Even in the presence of hardship and trouble, do they maintain their focus and clarity? These are the people who understand magic. They may not cast spells or do ritual, but they do focus their energy and intent properly. They are aware of how their thoughts, words and actions effect their life and the lives of others.