Shakedown is a command in the bottom right menu that orders all regular guards to search the entire prison for contraband. Guards will search prisoners, kitchens, ingredients and delivered boxes. A list of everything found will be listed at the top until the shakedown is complete. Prisoners found with contraband will be punished according to your prison policy.
Prisoners aren't fans of shake-downs, for obvious reasons. It is best to perform shake-downs while the prisoners are working or eating, not when they are sleeping! They don't like to be woken up in the middle of night with a guard in their cell searching them. Performing shake-downs too often will also annoy your prisoners and raise your prison's danger level, so try to make it a rare occasion. Armed Guards and Dog Handlers do not assist with shake-downs. They will continue with their patrol route or whatever they are currently doing.