Well the solenoid you can at least check roughly as it should give a realtively loud cklicking noise when operating. You can normally hear that inside the car while boosting. The pressure sensor you can get the idea if you compare the boost reading on your AVC-R with another source, i.e. an external boost gauge. If they are both fine and working and all pipes/hoses/connections are 100% ok then there's only the main unit left to be faulty. To be honest though, I doubt it, as so far I never heard of a main unit going kaput on the AVC-R. Only the display on it due to vibrations etc.
AVC-Rs are relatively hard to get at the moment. And they're not going for cheap either.