One concept suggested in the past was for a particular general Mode C squawk to indicate that a verified Mode C check had been carried out. But, of course, that would be too easy to abuse.
However, if the verifying unit issued a 'verification number' which the pilot then acknowledged, at least it would deter such abuse. E.g. the last 2 letters of the ICAO code plus the Julian date, then the next number in sequence plus the last 2 codes of the registration? So, for example, Brize Radar issuing a Mode C check to the 7th aircraft of the day, G-ABCD, on October 18th would issue 'verification VN291007CD'?
More work for ATC, perhaps - but probably worth it to have verified Mode C squawks generally available?