Do you have a diagram indicating the function of each tubing connection for this compressor?
The best way to tell is from the size of the lines. The smallest one will be the high pressure output. The other two are both i the low (suction) side. One is used to connect the compressor to the system and the other is the process tube for access into the sealed system. Which is which is dictated by the configuration of the sealed system. Usually the compressor will come with a manufacturers' sheet describing all specifications of the part.