The fittings for R-12 are threaded fittings, meaning the A/C guages will have to be 'screwed' onto the lines. R-134 fittings will not be threaded and will be larger. They will have a 'notch' or 'groove' around the fitting about half way down it. The low side fitting will be on the low/suction line or on the compressor head itself. If it's R-134, it will be the smaller of the two fittings. It can be 'retrofitted' to R-134 if it is set up for R-12 now fairly easily. But to diagnose the system, you'll need to hook some guages to it and look at the pressures. If you have guages attached, and the pressures are near 75psi or higher, and close to equal on the high and low sides, look at the compressor. Is the hub out at the belt turning? The very end of the compressor will be turning if the coil/clutch is engaged. If the pressures are very low when you attach the guages, then it's low or out of freon. The low pressure switch will prevent it from engaging. Hook some guages up and get back with me and we'll go from there. I love A/C work!