How to find an A/C evaporator core leak?

The inside of the windshield has an oily film, and the A/C cooling is inadequate.

An oil film on the windshield could be cause by a refrigerant leak in the evaporator core that may allow some refrigerant oil to escape into the evaporator case. A/C blower motor operation in the defrost mode could carry the oil through the ducts to the windshield. 

A plugged heater case drain would not cause an oily film on the windshield. An oily film could only be caused by a refrigerant leak or by a heater core leak. A plugged case drain could cause water to leak out onto the carpet or possible a musty smell coming from the vents.

A/C compressor oil.

How to add oil to the AC compressor ?

Modern compressors no longer have a dipstick to allow the compressor oil to be checked

All modern compressors require the oil to be drained from the compressor and measured in a graduated container.

If the system had a leak, the measured amount of oil may be lower than specifications. Always refer to the guidelines given by the manufacturer before adding oil.