hey whats up rob actually there isnt an exact science to it but there is a way to help you well to ways one is by looking at the dvd drive
if you lucky enough to have either a sammy or a benq your pretty much set but if you have an Hitachi drive well then things can get tricky and you would have to open your case which will void your warranty all Hitachi drives are hackable just the v79 drives are the hardest and require you to remove a piece off the drive

but if you do have an hitachi this might help determine which one its a list of all the xbox 360 that were made and when and sold to where it can help you figure out which one you have without having to open it up
http://www.360drives.com/list


just on a side note if you just recently bought or had a returned 360 and have a benq drive be careful the new benq drive is uncrackable as of yet but there are a few people working on it i would say check out 360mods.net