Mike is right, but if it is converted there is not always a label. If it simply has 1/4" inverted flare connections, it is most likely still R12. If it is 134, it will have quick disconnect looking connnection adapters.
An a/c shop will have a refrigerant identification instrument that they use before recovering refrigerant. This ensures that they do not contaminate their recovered R12 with alternative refrigerants.
There are many people using all sorts of off the wall "alternative" refrigerants. These things are blends of 12 and 134, propane, butane, and who knows what. It would be worthwhile to find a good a/c shop and have them test it and fix it for you.
There are many independent shops that are very good auto a/c shops. If it has not been converted, I would recommend that you stay with R12. It will cool better and the price is now coming down since many cars old enough for R12 are being taken off the road.