I'm not familar with your particular model, so I can't tell from the pictures. I can, however, trace the lines on any a/c system and tell which is high and which is low. You need to get an a/c book and do some reading on a/c fundamentals. Once you understand the principles you will have no trouble determining which is which.

I strongly suspect that your left picture is the high side and the right picture is the low. You can proceed safely on an R12 system by connecting your high side gauge to the one you suspect as the high side and start up the system, if the pressure goes up above 80 or 100 with the system running, then that is the high side. If not, connect the high side gauge to the other port and see if you have much lower pressure. You use the high side gauge for this test so that you don't peg a low side gauge, possibly damaging the gauge.

If the compressor is not kicking in, then it doesn't matter for your preliminary test because you will have the same pressure on both sides. If it is below about 83 or 84 psi, then the system is empty, actuating the low side cutout switch and that is why your clutch is not kicking in. That is the most likely scenario in the case where the clutch is not engaging.

Good luck,