Limited slip diff
 

Does my RX7 1st gen have it? Or, how can I tell?

What year and model is your RX-7? Or a more easy way to tell, Does your RX-7 have rear disc brakes?

If it does, it has a limited slip differential.

Disc brakes do not necessarily signal an LSD. A GSL or GSL-SE would have come from the factory with a limited slip, so if that is your trim level you have a good possibility.

Jack up the rear end so both tires are off the ground and rotate one of the wheels, if they both rotate the same direction then you have an LSD. I have had open diffs with enough crud and bind in them to mimic this, but 9/10 times you are good to go.

Isaac