I hope you don't take this the wrong way, but if you must ask this question you really should not be messing with it. If you understand refrigeration fundamentals, you can always determine which is high and which is low by tracing the component layout on even a "new to you" system.
To do it properly you should connect a manifold guage set. The set cannot be connected wrong. On the 134 system the high and low side are different, non-interconnectable fittings. At that point you must know which knob to turn to see that you are charging via the low side.
Please be careful, or better yet get someone with a manifold gauge set and proper knowledge to charge it for you. I don't want to see any of our mshop siblings at risk.
We used to have a frequent poster that went by: "Deezel" He was the self appointed "Safety Officer." I expect that he had seen some bad accidents in the past. He was constantly chiming in on threads with safety information of all types. He is missed.
I'm not near as dilligent as he was, but when I see red flags I chime in with safety warnings. I will take the chance of hurting someones feelings or being seen as rude, and in return hope that I can prevent someone from getting hurt.