The rear tires will spin in opposite directions on all open diffs. However, the same will occur with some limited slips as they only lock up when sufficient velocity difference exists between the two axles.
Short of pulling the rear cover, the best method for determining if you have limited slip or locking rear axle is the functional test: Find a paved road with a loose gravel shoulder. Pull off far enough to have one tire on pavement, the other in the loose gravel. From a stop, hit the pedal fairly hard. With a properly operating limited slip, you will have near-normal acceleration. A shower of gravel and very little progress indicates an open rear end (or, of course, a trashed posi!)
