I've had a ballot returned to me because my signature didn't match my on-file signature close enough.
You mail in two sealed envelopes... You sign the inner one, which is discarded once verified. You get a barcode with your ballot that you can use to verify that your ballot was counted. I remember reading a big 'voting transparency' page years ago, but I can't find anything today that documents everything.
So, knowing that signatures are checked feels good, even if it's a random sampling, they'd be able to gauge whether deeper auditing is required. Fraud would require intercepting mail, which means:
A) Intercepting received ballots.
B) Intercepting sent ballots.
For A: Ballots are delivered across the state in a very tight window. Prior to the ballots, voting pamphlets are sent to everyone, telling them when to expect their ballot. It is assumed that any meddling with the ballots on a large enough scale to affect the election would be reported well before the election.
For B: Fraud would occur in three ways: Trashing the ballots, modifying the ballots, replacing the ballots. Trashing would be identifiable by concerned citizens monitoring that their ballot was counted. Modifying would be identifiable by the security envelope that was signed by the individual. Replacing would be identifiable by matching signatures.
There certainly is an opportunity for Fraud, but the organization to pull off an attack that is wide-scale enough change an election would be big enough that it would likely set off other alarms.