There is no method for knowing. The Master simply reflects, just like the mirror. What is the method of the mirror for knowing your face? There is no method; the mirror reflects. If there is no dust on the mirror, it reflects perfectly. If there is too much dust, a layer of dust, then it doesn't reflect at all.
A Master is a consciousness without any dust, a consciousness without any content, a consciousness without thought process.
Thoughts are the layer of dust on your mind. It is because of thoughts, too many thoughts, that your reflections are not true to reality. They interfere, they continuously distort, they manipulate. They never allow the thing to be reflected as it is. Once thoughts have been dropped, once you have come to a point when your mind is contentless, you just are, no thought moving in you, then nothing is distorted. Then the lake of your consciousness is absolutely still, no waves, not even ripples. Then the moon and the stars - all are reflected in their great splendor. The lake becomes a mirror.
-Rajneesh