How do I identified the rear to b 12 bolt posi by bolts &inside

I I have a 68 Chevelle I bought it had a 350 built for it
with roller rockers it had a hydraulic 327 before that
motor not sure if it was original or not I have a rear for
my Chevelle at my ex's house and he keeps putting
me off on getting it he's stolen from me before how
do I tell if I'm getting my 12-bolt posi rear back do I
have to open up and look at the gears and look for
some specific things I'm not sure please help
thanksnot really positive bout body style its 2 dr hard
top chevelle not a ss