Easiest thing in the world. First you find out for sure that it is a Chevy rear end. Once you've done that, then count the bolts holding on the cover in the rear of the 3rd member. If there are 12 bolts, it's a 12 bolt rear end, 10 bolts means it's a 10 bolt rear end.
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them