Some of the following is excerpted from the NPCA Field Guide to Structural Pests, available through the NPCA(National Pest Control Association) Resource Center:
NPCA

The following list indicates common signs of a subterranean termite infestation. It is recommended that you have your house thoroughly inspected by a pest control professional trained to detect the sometimes subtle signs of termites.




 

• The presence of mud-like material which lines the galleries in an irregular pattern.
  
  

   

• Termites may excavate the wood so that only a very thin layer of wood is left on the surface between the cavity and the outside. Then this layer is broken, they will cover the holes with mud like material, used to make the tubes. This is a mixture of soil, feces, and saliva.

   

• Swarmers: The appearance of a swarm of what you would call "flying ants",especially near light sources This indicates a nest may be near. A "swarm" is a group of adult male and female reproductives that leave their nest to establish a new colony. Swarming occurs when a colony reaches a certain size. Swarming is most common in the spring (March, April, May, and June) and occasionally autumn (September and October).

      Most swarmers emerge during the day, most frequently on warm days after rain. Swarmers found outdoors near tree stumps, railroad ties, etc., are not an indication that your house is infested, but are present on the outside only. On the other hand, finding swarmers indoors often means that you have a termite infestation within your house. At first glance, swarmers and ants look similar, but can be distinguished by certain physical features. At Differences between ants and termites  there is a picture and list of differences.

   

   

• Mud Tunnels: Subterranean termites maintain their headquarters in the ground and build "mud tubes", pencil-size, that connect the nest (moisture) to the food source (wood).The tunnels may contain broken mud particles with fecal materials. Mud tubes or shelter tubes are proof of termite infestation, but their absence does not necessarily mean that a structure is free of termites. The insects may reach sills and other wood members through cracks or voids in the foundation wall, under the outside stucco, or from earth-filled porches, steps, terraces, or patios. You can break open tubes to determine if termites are still active inside. Termites often rebuild damaged tubes, another indication of continued activity. Old tubes are dry and will crumble easily.

  Subterranean termites construct four types of tubes or tunnels. Working tubes (left) are constructed from nests in the soil to wooden structures; they may travel up concrete or stone foundations. Exploratory and migratory tubes (center) arise from the soil but do not connect to wood structures. Drop tubes (right) extend from wooden structures back to the soil.

   

  

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• Piles of wings: The shed wings of swarmers indicate termites have entered their next phase of development.

   

• Live termites: Reproductive kings and queens are 1/2" long, winged and black or brown in color. Workers are sterile and usually hidden within infested wood. They are 1/4" long, wingless and white. However, the absence of finding live termites does not mean that they are not present in the structure.

   

• Buckling paint or tiny holes on wood.

   

• Damaged wood: Wood may appear crushed at structural bearing points. Termite damaged wood resonates with a dull thud (hollow sound)when tapped with a hammer. Pick and probe the surface of an infested piece of wood with a pen knife and you will find tunnels running parallel to the wood's grain.

  Finding termites in a structure does not mean you have an immediate emergency because the rate at which damage occurs is relatively slow.