How do I know that I need the timing chain fixed?

You won't know, usually the Timing Chain is good indefinitely unless your engine starts running bad and it is because the Chain is stretched and your engine jumped a tooth on the gear and now the timing is off.

Either your car won't start, or if it does it will backfire and sputter. Then check your timing when the timing mark gets dead center on the flywheel, your number one piston should be at the top of its stroke, you can pull the plugs and check this with a pencil inserted in the spark plug hole!