I always check RWD Volvos for a limited slip diff. I found a scrapped 265 years ago with one. The "guts" now reside in my 144S along with 4.30 gears from a scrapped 142 that had factory OD and therefore the "lower" gears.
To check (limited slip--not "lockup) -- rotate one rear wheel by hand -- if the other side turns in the same direction you've lucked out!
Lock-up rears (in late model 940 cars) will spin like a normal open diff with hand spinning (opposite wheel soinning in the opposite direction) -- they need to be spun fast enough (by the motor I would think) for the lock-up to work. Just ask me how I know this :-) -- Dave