Finding a refrigerant leak
One of the most common places to look is the discharge hose from the compressor, it is very hot from the heat of compression and this sometimes causes it to deteriorate first in some systems. Look along it's length for signs of oil, this line runs from the compressor to the condenser coil in-front of the radiator. All oring fittings should be given a visual check for oil leakage throughout the system. Look at the hood inside, is there a visual line on it in-line with the compressor pulley, this could indicate a compressor seal leaking.
If none are found, the default place is where you can't see, the evaporator coil inside the dash.