You should replace your timing belt at the manufacturer's suggested intervals regardless of its visual condition, but it's a good idea to do an inspection every 10,000 miles or so. On most cars the timing belt is easily seen by removing the plastic timing cover on the front of the engine, usually held on by a couple of phillips head screws or clips.
To inspect the belt, first look at the outside of the belt to see if any tiny cracks are forming. One or two small cracks in the outer shiny coating of the belt are ok, but if you see lots of cracking on the surface this may indicate extreme wear. Next flip the belt over slightly to inspect the teeth. A single broken tooth can be catastrophic. Also check the belt's play by twisting it. If you can turn it much more than halfway around, it might have too much free play. Check your manual to see what your car's specs indicate.
Don't hold off on timing belt replacement. If it breaks or shreds, you can be looking at some serious repair bills.