I have been told that when a car's A/C system has been converted to R-134a that there is supposed to be a clearly visible label placed on the compressor. One of my cars has been professionally converted in the past and it does have that big "R-134A" sticker on it.
You might ask to look at the receipt for that service. And if you have a chance take a look at the A/C compressor under the hood and see if it has a conversion sticker on it.
The highest probability is that a professional place put the right Freon in the system.
Now for the important question:
What color is it exterior and interior?