The most common sign of a termite infestation is the distinctive mud tube. These tubes come out of the ground usually against a wall or other supporting object. In basement and crawl spaces where the rafters are at ground level the tubes are often seen coming out of the wood on the sill plate , band board or joist.
Their purpose is to provide the termite colony with a climate controlled tunnel (termites like to stay in the dark) from their “home” underground to the food source. In the case of a home the “food source” is usually a part of the structure that is made of wood. If these tubes are hard and dry, they are not in use. If they have a moist feel, most likely termites are using them.
Typically you will not know that termites have invaded the structure until they have been there for a while. They tend to eat the center out of a piece of wood, leaving the exterior intact. If the damage is visible you may presume that the infestation is serious, and that the possibility exists that the structural integrity of that area of the building has been compromised.
Swarms of what appear to be flying ants in the late spring should be checked out immediately. This swarm may be a colony of termites.