[rurp]

I worked as a cashier at a place that sold liquor and we would occasionally get decoys. It was usually the same young guy who would try to purchase a single beer, without saying a word.

If the cashier asked for his i.d., the guy would silently hand them card that said something like, "Congratulations, you did not fail the underage liquor sales test", and then leave.

After the 4th or 5th time of the same person trying the same purchase most of the employees learned who he was. I think law enforcement did eventually switch to a different decoy towards the end of my time at the job.