The only way to know with absolute certainty what type of refrigerant you have would be to have it professionally anaylzed. Even then, I wonder just how many shops are trained sufficiently to come up with a chemical breakdown of a cars refrigerant?
A shop would most likely look at your fittings for determination as the R-12 and R134A fittings are a different size.
There are all sorts of "blends" on the market that can be fed into an R-12 system and unless you know the complete history of the cars maint., you could have a "mixed bag".
Don't mean to be an alarmist, just pointing out some possibilities.