to show the cloven foot

from The Collaborative International Dictionary of English v.0.48

Cloven \Clo"ven\ (kl[=o]"v'n), p. p. & a.
   from {Cleave}, v. t.
   [1913 Webster]

   {To show the cloven foot} or {To show the cloven hoof}, to
      reveal a devilish character, or betray an evil purpose,
      notwithstanding disguises, -- Satan being represented
      dramatically and symbolically as having cloven hoofs.
      [1913 Webster] Cloven-footed