How do you locate a refrigerant leak on residential air conditioner?

Look at all of the joints and pressure control connections. A leaking connection will have signs of oil around it. Due to the fact you are losing compressor oil along with refrigerant at the site of the leak. Spray a soapy water mixture on the joints until you find one that shows bubbles, they may be very faint and hard to see but if they are there that is a leak. Make sure the flare caps on your gauge ports are on tight and if you have the ones that are put on hand tight look inside the cap to see that they have the rubber gaskets intact.