Try taking a digital pic of the said mirror ( close enough that you could see a face ) and then look for any faces in the mirror pic. Just make sure your not in the pic also. You can ground yourself by going out side bare foot and come in contact with the Earth. P.S. let me know if you see any faces in your mirror.

See the link below and look for the mirror pic, it says that faces appear in the pic. It's from the Colorado Black Forrest haunting.