Is there a sure fire method to determine if my AC was ever converted to R134. I have a few cans of R12 and would like to charge my system. The problem is I have no Idea what was done in the past.

I have the older Compressor style I think a Delco, last year I jumped the compressor clutch and could hear a hissing under the dash on the passenger side. I want to check for leaks but would hate to use up the R12 for that purpose.

77 300D