A B or C is easy...
'A' has a flange output, Porsche synchros and the old monkey-motion shifter (and removable bell housing). Only seen in early Zeds and sports 2000's.
'B' is the common box used from 1971-1986 (or thereabouts) has the better shift mechanism, Borg Warner-style synchros, yoke output, and comes in many ratios (since it was fitted in the 260Z -> 910 Bluebird and everything in between), plus there are some 'upside-down' shift pattern models - these are 'Option' or close ratio boxes (hard to find for sale these days). Also comes as a 'stumpy' in the R30 Skyline.
The 'C' series has the bolt-on shifter plate, twin synchros and is as-fitted to all RB's, SR's, CA (twincams), etc, etc.
There are different sizes of gearsets in the 'B' (e.g. FJ20ET has wider gears than FJ20E) and the 'C' comes with different casings depending on the engine (the RB25DET box is larger than the RB20DET box).
What you probably want to know is the ratios of the box you've found (assuming it's a 'B') - and the only way to tell for sure is to measure them yourself. A couple of texta marks on the input and output shafts, then start counting!