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, wherein the set of images are obtained during a speaking session in which the subject is instructed to speak; in at least two images of the set of images, automatically detecting a straight line in each image, wherein the straight line is a symmetry line of the face, wherein the symmetry line intersects at least a mouth region of the face, wherein the symmetry line comprises a plurality of pixels representing the mouth and at least one pixel below the mouth and one pixel above the mouth; and automatically verifying that the subject moved the mouth to speak during the speaking session, 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 an opening of the mouth between the pair of images; and determining the score is above a threshold, wherein said computing the score comprises computing an integral of a function along a range, wherein the function comprises multiplying a difference between a value in a first symmetry line and a statistical measurement of the first symmetry line and a difference between a value in a second symmetry line and a statistical measurement of the second symmetry line.