1. "Swarmers". When a colony exceeds a certain population, swarming occurs. Winged swarmers appearing in large numbers, especially near windows or light sources, indicate a nearby nest. These swarmers are a group of adult "reproductives" that have left the termite nest to establish a new one. Swarming behavior is most common in spring (March, April, May, and June) and early autumn (September and October), especially after warm, rainy days.
If you notice swarmers outside, it does not mean your home is infested, since they are most likely coming from an outdoor nest. Finding swarmers inside though should alert you to a growing infestation within your home. (It is not uncommon for these winged reproductives to be mistaken for "flying ants". Read the article about the difference between termites and carpenter ants.)
2. Mud Tunnels. Subterranean termites will often make their nest in the soil (moisture source) and then build highways called "mud tubes" that run vertically or otherwise to connect the nest to a wooden food source. Mud tubes are a definite sign of termite infestation, but the absence of mud tubes does not mean that no infestations exist, since there are other ways that termites reach food sources. Mud tubes can be broken open and inspected for live termites to determine whether the tunnel is active. If it is old, it will be dry and easily crumble.
3. Piles of wings. Before swarmers enter the next stage of development, they will shed their wings which are often left in scattered piles near windows or light sources.
4. Live termites. There are four different kinds of Subterranean termites:
Primary Reproductives- Black to pale yellow-brown, pale wings with few visible veins; ¼ to 3/8 inch long.
Secondary Reproductives- White to cream in color; wingless, or very short wings
Workers- white in color, wingless, and ¼ to 3/8 inch long.
Soldiers- Resemble workers in overall appearance, except with a slightly larger brownish head and more prominent mandibles or jaws.
5. Damaged wood. Wood with sustained termite damage might look "crushed" at structural joints. If you tap the damaged wood with a hammer, you will hear a dull thud. Wood suspected of termite damaged can be further inspected by probing the surface with a screwdriver or pocket knife to expose tunnels. Subterranean termites excavate tunnels that run parallel to the grain.
6. Wood Galleries of Subterranean termites are lined with a mud-like material set in an irregular pattern, and an extremely thin layer of wood between the gallery and the outside wall.