How do you determine which freon to use in a/c

Your car was made on the brink of the changeover. All 1994 models use R134a, most 1993 models used R12. However, if you are not the original owner of the car, it may have been converted to use 134a, as R12 is no longer manufactured and is very difficult to find (and expensive). If your Buick has been converted, the R134a fittings will snap right on. They will not fit the R12 ports, completely different. If it has not been converted to R134a, you should buy a retrofit kit, (not expensive) and install it. Suggest you go to an auto parts store (O'Reilly's, AutoZone, Advance), and ask them to take a quick look under the hood, and tell you whether the retrofit has been done. (It's a very simple job.)