I am under the impression that some cars have timing belts that need replacement at about 100K miles and some don't. I am looking at a 1999 Ford Escort with that many miles and don't know if it's been replaced - or if it si not a car with a timing belt. Can someone fill me in. Also, is it a car that would sustain serious damage if one waits until the belt fails before repacement? Thank you.

Yes, you have a belt. What I read was to inpect every 120K, replace as required. But, what happens when the belt fails while driving... could internal engine damage result?
At the mileage you're at now, it would be cheap insurance to replace it and enjoy the peace of mind.