There are too many self proclaimed "Masters" out there making a very nice living, it must be said. However many of them are of low ability and their form has a high "wushu" content. Of the good ones, you can see the energy and the strength of their form, often very understated. And as Xue Sheng says, push with them and there is no doubt about their skill. My own teacher prefers to be called "Coach" but there is no doubt in my mind about his "Master" status. Only experience can help you determine the difference. Having been duped several times in my Taiji journey and gone through the door with the bright lights and sparkly advertising, only to find when I got through it, the room was empty.
It is often said "You may not find a Master by looking, rather the Master will find you". this was so in my own case.
Very best wishes