To diagnose a leak, buy freon containing UV detector dye and scan with a UV light. The cans are about $20.
This is probably unneeded in my case as the bottom of my compressor is pretty wet with oil. I assume it is refrigerant oil accumulating as my freon escapes and sunburns the polar bears. Not really, we just call it freon. It's really inefficient but environmentally correct R@#& which required MB to install those huge electric fans in front of your radiator in addition to your belt driven fan.