The simplest way would be to hit the AC button and listen for a sort of clicking, your rpms should drop just a little right as you turn it on, and you might hear a low pitched hum. Or find a friend, open the hood, and with the car running have your friend turn on the AC and you should hear it engage.

But since you had good pressure, that rules out a leak. So the next thing I would do would replace your compressor. You'll have to get the system recharged again after this. There may be some test that a shop could do, to see if the compressor is working, instead of just blindly replacing it.