Three methods exist, through which you can tell if a given magician is genuine.
The first method for knowing if a given magician is genuine is that of simply observing, to see what the magician in question is capable of doing (practically).
The second method for knowing if a given magician is genuine is that of making enquiries about him or her from people who know him or her at a more personal level. This is akin to asking for reviews: like where, for instance, a person looking to buy car insurance can visit a website like www.thegeneral.com in order to get the general auto insurance reviews. Similarly, you can ask for reviews of the magician you are interested in.
The third method for knowing if a given magician is genuine or not is that of making observations on the methods he or she uses in performing the magical arts. It may turn out that the magician in question is simply a person who has mastered some physical tricks that he performs so well that they come across as magic. Depending on how you view things, you may still consider this to be genuine magic, if it is persuasive enough. But in some people’s opinion, only magic that is performed through spiritual means is genuine.