He will have a mole on his forehead

Abu Basir quotes Imam Muhammad Baqir (as) or Ja'far as-Sadiq (as) (the uncertainty stems from the narrator) that he said: "There are two signs of the Qaim (as). There will be a mark and a mole on the head and a mole between the shoulders.
(Sheikh Muhammad bin Ibrahim Nomani, al-Ghaybah al-Nomani, p. 253)