On the rear axle there will be a 3 digit number stamped into the diff housing somewhere, which will identify the diff type, other than that you'll need to measure it to find out exact details.
Are you wanting to ID a trans and axle that you have already?
Cheers guys, finally know what i'm looking at. I have got a 71/4 rear axle with a lsd in it, but heard they ain't that good. Know where there is a narrowed 83/4 with moser shafts and a spool in it, four link barackets, any good, might buy it cos its going cheap.
7 1/4 is ok for near stock 318 use, if I remember right people have run 13s on them without problems ... a built 8 3/4 will be up to serious abuse though, lol.