For a quick ID of in-service trucks -
M939 (often referred to on here as M939A0 for clarity)- NHC250 engine, 11.00s
M939A1 - NHC250 and 14.00s
M939A2 - 6CTA 8.3 engine, 14.00s, CTIS
Post service, many owners have put 14.00s on A0s, and 11.00s on A1/2s, to suit their personal needs/desire, so it isn't a hard rule.
If the truck has a winch, it will have a slightly different designation, rather than just W/W added (as in M813A1 W/W). M931A2 is a tractor without a winch, M932A2 is the same tractor with a winch on it.
Again, sometimes they have been retrofitted, or removed, so it's a general way to identify. For some reason, the Marines had quite a few winchless trucks with M932 data plates.
Cheers