The low-pressure port itself is usually smaller than other fittings on the A/C system and will have a Schrader valve (similar to a tire valve) that you can access with an A/C manifold gauge set.
Important Note: Because you're working with refrigerant, which is under pressure and potentially harmful, I strongly recommend you do not attempt this yourself unless you have experience working with automotive A/C systems and the proper safety equipment. Refrigerant leaks can be dangerous to your health and the environment. Improper handling can also damage the A/C system further. It's best to take your vehicle to a qualified mechanic or A/C specialist for diagnosis and repair.
