I really don't know if it was an option or not, but the 88 Owners manual says "Theft Deterent System - U.S.A. Only". There should be a Security light to the far right of the gauge cluster if you have it.
The theft deterrent computer is underneath the driver's side dash somewhere.....I now have a 90 Canadian model Sup that is still in the middle of a 1JZ swap (finishing some wiring). I have heard that they did not have factory alarms, but will check mine tonight and let ya know what I see. I had planned to install an aftermarket one anyway....