It is rare our timing belts fail, but when they do, here's a very good illustrated guide for troubleshooting timing belt failures:

http://www.carquest.com/product/belt...ing_belts.html

I was browsing eBay and noticed a timing belt vendor had "borrowed" this guide from the CarQuest web page. Here's the first few examples, so you can see what it is like (there are 13 other examples on the referenced web page):


Tensile (Cord) Failure - Inspect for:

Belt crimped
Foreign body in drive
Excessive tension
Moisture or antifreeze on belt invading to the cord


Timing Belt Teeth Sheared
Inspect for:

Seized drive component(s)
Improper tension
Misalignment