relative temperatures of the various parts of the system.  Things should get cold at the ex valve.   Should have one hot line and one cool line on the compressor.   

There comes a point when you have to take things apart and check components.   It's either the compressor not making pressure, or the expansion valve not closing enough to maintain it.  Based on my experience I would start out suspecting the ex valve because compressors usually tend to fail more magnificently then just not pumping, and when they do, it usually causes a plugged drier which can be detected by temperatures.