As a general rule, we only allow the kids to have one dessert a day (if that, many days they don't get dessert at all). Most of the time, they get a cookie or a small piece of candy after dinner, but if they spend the day with either set of our parents, we know that they'll get their daily serving of dessert there. It's a given: grandma's house = dessert.
So when the boys spent Friday with my mom, we just knew they'd had something sweet. But after dinner (back at our house), Liam asked for dessert. Our conversation went something like this:
Liam: I want dessert, please.
Me: Did you have dessert at Gram's house?
Liam (avoiding eye contact): No.
Ryan: You didn't have a cookie?
Me: You didn't have a popsicle?
Ryan: You didn't have ice cream?
Ryan: What did Evan have for dessert?
Liam: A cookie.
Me: Evan, did you have a cookie at Gram's house?
Me: Evan, did Liam have a cookie, too?
Aha! We've now found a fail-proof lie detector test for Liam (age 3). At least until Evan (age 2) turns three and learns how to lie, too.