Most LSD rears will spin both tires the same direction if you jack it up and turn them by hand. Also if it is in gear, they'll probably just be locked. An open or 'one legger' will let you just spin one wheel while jacked up, and the opposite may spin in the opposite direction. But yes, the only real test is to take the cover off and look at what is in there. The grass test is a decent one if you dont want to go thru all that work. It's not foolproof though; one tire on wet grass, one on dry pavement... Grass and LSD seem to go together just fine.
