Reveal and counter-reveal
The scenario of the real Sheriff
"On the first night I checked no. 4: Irish, and no. 9: the city. On the second I checked no. 6: Italian. I suggest we discuss no. 4 and no. 6. The rest are my hypotheses."
Testing the claim
Check the number of checks, the timeline of speeches, and that a suspicion has not been passed off as a check.
Two Sheriffs
Compare both versions by the same criteria; do not automatically choose the more confident one.
The false Sheriff
A mafioso announces two "checks" and accuses a competitor. The real Sheriff chooses between an immediate objection and the risk of being exposed. The city checks whether the statements fit the number of nights that have passed, and does not accept the two versions as equally reliable just because they are delivered confidently.