To find it definitively, you should:
1. Consult your vehicle's repair manual: This will provide diagrams and pinpoint the location precisely.
2. Look closely at the compressor: The low-pressure port is usually identifiable by its size (smaller) and potentially by a label or color-coding near the port (though this is not always present). The high-pressure port is typically larger and thicker.
3. Search online for images: A Google Images search for "1998 Buick Park Avenue AC low pressure port" might show you pictures from other owners' vehicles, giving you a visual reference.
Safety Warning: Remember that the A/C system contains refrigerant under pressure. Working on an A/C system requires special tools and knowledge to avoid personal injury and environmental damage. If you're not comfortable working on the AC system, take it to a qualified mechanic.