To be certain:
* Consult your owner's manual: It should have a diagram showing the location of the AC ports.
* Look for a smaller, blue cap: The low-side port often has a blue cap to distinguish it from the high-side port (usually red or yellow).
* Use a diagram specific to your year, make and model: Online automotive repair databases (like AllDataDIY or Mitchell 1) often have detailed diagrams showing the AC system components.
Be cautious when working with refrigerant. It's a hazardous substance, and improper handling can cause injury. If you're not comfortable working on your AC system, it's best to take it to a qualified mechanic.