Jack it up and spin the tires if they rotate opposite of each other it's an open diff. It they spin the same direction it's limited slip, sure grip or posi or whatever term you want to call it.
In neutral.... and spin by hand. (Both rear tires off the ground)
I agree with above. Pull the cover off to change the fluid, and find out for sure whether limited slip or not. (If you don't already know otherwise... 95% chance it's open.)