Again, hoping nobody's said this:
Go into an empty check-out lane either as the clerk or as a customer.
As clerk: When you get customers, ask them obnoxious questions, like "Are you single?" or "Hey there gorgeous, would you like a free autograph?" and give them ridiculous costs for their food. (EX: "Those bananas will be $99.99, please!")
As customer: Drum your fingers on the counter impatiently. After a few minutes, start yelling loudly, "I'm over here, you morons! Come and pay me!" Then, if nobody pays attention to you, start having a breakdown and yell, "Nobody loves me!"