You know its bad when you hear it knocking or rattling you suppose to change it every 100,000 miles or so or if you want to stay on top of it change it every 5000 miles or so that's how I did I have a 1995 rodeo with 375,000 and never once got a new engine yet I get my oils up tune up on time and if you drive a lot that will do it my timing belt went up on my truck that's how I know it makes them knocking and rattling noises
I am assuming that you mean it won't start when the engine is hot. Assuming that it has done this more than twice and it ran good until it got hot I do not believe it would be a timing belt. For around $35 you can purchase a timing light and an autoparts store will tell you the procedure for your vehicle on how to check it. For around the same price a mechanic will check it once. There are various things that could cause this problem; start with a compression check, then look for fuel and spark. If all those are good it could possibly be a faulty sensor throwing off you timing. A Mechanic can check all this for anywhere from free to $80. I wouldn't pay more than $60. Good Luck!