Small gauge on the big pipe, big gauge on the small pipe outside at the condenser. The refrigerant type does not matter. Be careful when connecting and disconnecting the high side gauge. You can seriously burn yourself and also lose a significant amount of the charge and wind up needing to place a service call to have that corrected. I would ask what you hope to achieve by doing this, since whatever the gauges tell you is meaningless if you need to ask this question.