How do you find a refrigerant leak in a refrigerator?

I have trouble shot the fridge to the point that I know it's not the defrost heater, defrost timer, or compressor.

I have pulled the back of the freezer off and there is no blockage, no ice, no frost, no cold at all... even at the restrictor. There is still some oil in the system and I can hear it gurgling in the low pressure side.

I have not seen any oil on any of the copper. there is some corrosion in the lower portions of the tubing, but nothing that looks like a leak to me.