Termites create tunnels in wood. They prefer soft grain, spring growth wood or damaged and decaying wood. They will feed on dry lumber as long as the moisture from the ground is adequate. To increase the moisture level in the wood the workers may bring mud up from the soil and pack it in the tunnels. The presence of mud in the tunnels indicates the tunnels were made by termites and not by carpenter ants.
Carpenter ants leave clean, smooth tunnels.
The absence of mud does not eliminate the possibility of termites. They don't always pack it in the tunnels. Termites may enter a building in a number of ways. Wood or siding that is in contact with the soil can provide direct access as can cracks in the foundation. Brick, stone veneer or stucco over foam in contact with the ground can also provide hidden access.