Whenever I pay with a $50 or ($100 bill) I always say, “Is it okay if I give you a 50?” or “I only have a fifty—is that okay?” so they know what I gave them in an innocent, not so obvious way that I’m keeping you honest.
I sometimes do it with a $20 especially if it’s in a drive-thru.
I once was shortchanged at a Rite Aid. When I told the clerk she owed me $5 the manager had to come over and audit her register. I was happy it showed her exactly $5 over.
That is a great idea, I usually let them know big bills but for some reason didn’t that day.