I've been very lucky in my career in that I've only had two transactions that were affected by the discovery of termite damage. One of the transactions was canceled completely and the other was complicated by a very costly repair. What's the worst part? In both cases, the homes were occupied with otherwise responsible residents who could have spotted the damage much earlier on and saved themselves thousands of dollars if they would have been familiar with the signs of a termite infestation.
I have a couple of online resources that will help you learn how to spot a termite problem. The first is a concise article from Terminix that outlines the basics. The second is a photo page with some pretty good shots of typical damage.
http://www.terminix.com/Exterminator/FAQs/Signs_of_Termite_Damage/
http://unexco.com/gallery/