im not sure which is high and which is low, but one of them is on the front of the compressor and the other one is on the line that runs out of the back of the compressor that crooses the front of the motor. it has the red cap on it (at least on my car).
The red & blue caps usually mean that a R134 conversion has been done. The blue cap is the low pressure service port and the red cap is the high pressure service port. The connections are a quick release coupling for R134. The R12 service ports used a threaded flare fitting.
Are you planing a R134a conversion or recharge of an disassembled or empty system?