Your headers will be crimped at the end right at the flange to restrict air flow. Aftermarket headers are compleatly round and are usually stamped w/ the companys name on it such as BBK.
The Stock H Pipe has 4 cats on it, aftermarket H Pipes have either 2 cats or non at all.
The cat back is pretty easy, it should sound stock or just look on the mufflers, the company name is usually stamped on them as well. Also aftermarket tail pipes ( which people usually get w/ an aftermarket system unless they just buy the mufflers) are 2 1/2 Inches.