Often shouted in a public place (party, bus, etc) when couples are together. People who do not have significant others will shout this to keep themselves occupied. The couples must then raise their hands to show that they were not doing anything other than cuddling. It doesn't work. Trust me.
*Mo and Flo are at a party, hugging*
Joe: Yo, Mo and Flo! Hand check!
*Mo stops fingering Flo. Both raise hands into air*
Mo: See? We weren't doin' nothing!
"Hand checks" occur when a Mexican, typically a Luis, has had his or her hand or hands in his or her pants for an elongated amount of time. By screaming "hand check," the Mexican is forced to immediately remove his or her hands to show that no masturbation has taken place.
Davo: Hey, get your hands out of there! Hand check!
Luis, the Mexican, removes his hands from his pants.
Davo: What's that gooey yellow stuff?