Because termites eat wood from the inside, you usually won't see surface damage. You may see dried mud tubes and a few other dabs of mud here and there (possibly). A termite inspector may be able to tap a wood beam and detect a low or flat sound that indicates that the wood is somewhat hollower on the inside than it should be. This can be a subtle distinction a novice may miss, though.
In the advanced stages of disintegration, you'll see little bits of mud inside termite damaged wood. You'll also notice it has been excavated leaving long, deep parallel grooves. That's because termites like to eat the soft spring-growth sections of the wood and leave the hard wood portions alone. These striations will help you distinguish termite damage from dry rot or water damage.