Because they eat the wood from inside out, they can be undetectable until the beams are so weakened that you fall through the floor. It may not come to that, but if you suspect termites, call an exterminator. Termite control is often easier than controlling carpenter ants because exterminators can get to them in the ground or in the wood and take steps to keep them out. There are many ways to do this, including a relatively new treatment called Sentricon, which is a series of baits put in the ground to attract, then poison, termites. This is done by professionals.
Treatments can be expensive, and some are used in combination with others.
It is called Spectracide Terminate Termite Home Defense System, sold in some hardware chains and Home Depot, which uses a series of baits laced with poison that are put in the ground. Beware termites Termites swarm in the spring, just as ants do. If you see a winged termite, it does not mean you have them, but it wouldn't hurt to check the house anyway. The battle against termites is not just clobbering them where they live and eat. It involves keeping them out of your house, something the homeowner can do.