different cars deploy different valet protection.

most have a separate key that doesnt allow access to things like the globebox.
others are activated via the entertainment system.
some will reduce the engine power available.
cars that dont have the feature stock can have it added through aftermarket computer-reflash (like from APR for the VW-group)

ive read that some people stick a post-it on the cluster indicating the drop-off mileage, to conspicuously signal that "im tracking it"

personally, unless you have a really, really, really unique car, i just think valets handle enough machinery for it to become a chore to them, and not worth taking it out for extra miles instead of returning to the station to collect a tip. and things like the airport circuit isnt really conducive to them running wide-open throttle on a cold motor.

that said, i simply dont want any strangers handling my car, so i dont valet at all