There are several kinds of termites, including drywood termites, subterrainian ones, and even the most concerning, the Formosan termite, which will eat a tremendous variety of structural material - including asphalt. Any presence of termites is dangerous, as by the time you see them, they're usually part of an infestation. The exception to this is when they swarm to create new colonies, usually in the spring. At this point they are winged, and often mistaken for flying ants. Even though this may not mean an infestation of your property, if they are coming from your property this is a likely time you will see them.
Other evidence of termite infestation, once it reaches a certain point, can be fairly obvious - wooden structural material fairly shredded, the 'mud tunnels' they build to move around, etc. As with all pests, especially with insects, proper identification is critical before any treatment is determined. Roughly 90% of termite populations in the US are subterranean, with almost all the remainder being drywood. Once the species is identified, we have several very effective solutions to apply to your problem.
An arboreal termite nest we found, here in Albuquerque.
These are termite tunnels, which they build to protect themselves from predators - a sure sign of termites
Termite damage we found on a recent call.
On the surface, a termite infection might look moderate, but the insides of this house on the east coast have been obliterated.