1. A computer-implemented method comprising: obtaining a set of images of a face of a subject having a mouth, wherein the set of images are used for facial recognition of the subject; in at least two images of the set of images, automatically detecting a symmetry line of the face, wherein the symmetry line intersects at least a mouth region of the face; and automatically verifying that the subject moved the mouth, wherein said verifying comprises: computing a score based on values of a pair of images in the symmetry lines, wherein the score is indicative to a difference in the shape of the mouth between the pair of images; and determining the score is above a threshold.