I wrote the review of the Classic One MkIII in Mono mode for Dagogo.com, and if I recall correctly the MkIII is distinguishable from the previous iterations by the plastic encased binding posts. If the unit does not have the plastic on the binding posts it is not a MkIII.
To be certain you could obtain the serial number from the unit via the seller, and check with Pathos. A legitimate seller should have no problem with assisting in doing so.