Gut Check is an area that is located in Bowser's body in Mario & Luigi: Bowser's Inside Story. It is located right below Toad Square and leads to other areas in Bowser's body including the Energy Hold, Lumbar Nook, and the Rump Command.
Mario and Luigi must head into the Gut Check when Bowser is being challenged by a Wiggler to a carrot eating contest. Mario and Luigi must shoot all the carrot bits in the given time in order to win. Special colored orbs called enzymes can be shoot and which either makes the carrot bits smaller or completely destroys them.
Mario and Luigi also return to the Gut Check for the second time after Bowser (who was trapped in a safe at the time) swallows the second Star Cure.