Limited slip was not an option for a '98 SE.
There are a couple ways to verify if a limited slip was installed, either by a previous owner or maybe even a factory mistake:
1. Raise the rear tires off the ground. Spin one tire and watch how the other tire turns If the other tire turns in the same direction, you have a functional limited slip or a locker. If the other tire turns opposite or remains still, you have an open diff or a limited slip that is no longer functional. Sometimes you have to put a little resistance on the opposite tire in order for the limited slip clutches to grab and turn the opposite tire in the same direction.
2. The best way to know for sure is to open the diff cover and look at the shape of the diff case.
Open on left, Limited slip on right: