This picture shows carpenter ant damage to some 2x4's. Notice the tunnels in the wood, called galleries. Carpenter ant galleries are clean. As these ants excavate the wood, they kick out sawdust-type material, called frass, thereby leaving the galleries clean and smooth.
Also notice on the bottom piece of wood, there are openings in the wood. These are called windows and allow the carpenter ants to either enter or exit the galleries or remove debris like frass. On exposed beams, these window may be more easily seen than the galleries themselves.