Apparently corner of the eyes near the nose can give away the mask. If not fitted exactly perfectly the mask gives the appearance of a sort of double eyelid right at that location. Look there, and if it looks weird it is probably a mask.

Also, weird stuff around the ears such as them being too thick etc. can result in the ear-over-ear of some of the masks. Many masks may not go over the ear, and in that case the ear profile may be used to identify who it actually is under the mask, as ear shapes and contours are pretty unique.