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I believe that an open diff will spin the tire the opposite direction, while LSD will spin both tires the same direction. However, the stock MR2 parts use a viscous LSD, so at speeds you can acheive with your hand, it tends to act more like an open diff, since the viscous fluid requires quite a bit of pressure to apply any real force to the other side. Clutch-types and probably helical will spin both the same direction at by-hand speeds, as will a locked or welded diff. Also, I think if both wheels are spinning the same direction, it will make the driveshaft and/or output shaft spin, so you can't do it (with normal human strength) when the car is in gear.
Another indicator of LSD is when you go around tight corners and one wheel makes skidding noises; because one wheel normally turns faster than the other through a corner, if you make them spin at the same rate one has to skip to make up for the distance. It's not a sure-fire indicator though, because open diffs occasionally do that; my friend's '69 Mustang with 11½" rear tires skips like it has LSD, which we're assuming is the outside of each wide tire catching up to the inside of the tire. Edit: And the lack of skipping isn't sure-fire either, because weaker LSDs, like the stock MR2 ones, have enough give to let the car behave normally through low-throttle corners.
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