I dont believe the system needs an initial pressure in order for the compressor to engage - it will rotate whether or not its connected to the rest of the system (as long as the clutch is energized through the connector).
But getting back to the original post, if you are unsure if the AC compressor is turning, with the engine running and the AC off, pop the hood open and watch the AC compressor - notice that its pulley is rotating but the center is not. Now turn on the AC and again, watch the compressor - the center should be rotating along with the pulley. If its not, stop the engine, pull the compressor's connector and check for voltage (should be about 12v with the AC on