  We discuss the possibility of observing multi-lepton signals at the Large
Hadron Collider (LHC) from the production and decay of heavy Standard Model
(SM) singlet neutrinos added in extensions of SM to explain the observed light
neutrino masses by seesaw mechanism. In particular, we analyze two `smoking
gun' signals depending on the Dirac or Majorana nature of the heavy neutrino:
(i) for Majorana case, the same-sign di-lepton signal which can be used as a
probe of lepton-number violation, and (ii) for Dirac case, the tri-lepton
signal which conserves lepton number but may violate lepton flavor. Within a
minimal Left-Right symmetric framework in which these additional neutrino
states arise naturally, we find that in both cases, the signals can be
identified with virtually no background beyond a TeV, and the heavy gauge boson
W_R can be discovered in this process. This analysis also provides a direct way
to probe the nature of seesaw physics involving the SM singlets at TeV scale,
and in particular, to distinguish type-I seesaw with purely Majorana heavy
neutrinos from inverse seesaw with pseudo-Dirac counterparts.
