Y'all are doing good ones, but one easy way is just to ask how people are doing. Y'know? Like, if your a cashier. Just ask how the customer is doing or how their day is going. They will almost certainly say "good" but I bet you'd get at least a half or more of people actually having a shitty or neutral day and furthermore, knowing they're not having a good day.

If we made it so you can only get $100 per person max and they have to be knowingly lieing thatd be the way to go.