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Since I am somewhat new to working on a/c I can probably put it in laymans terms, but I have to put in a disclaimer first.
If you are going to work on the a/c system you must be fairly sure of what you are doing - even if it is the first time; otherwise, you might make a mistake and have a can of r134 blow up in your face. Also, there are only a limited number of things you can do without a vacuum pump ($200) and a set of a/c pressure gauges ($100) so keep that in mind.
Anyhow, to answer to your question. With the engine turned OFF and cold, put your hand on the flat part of the pulley just in front of where the belt is attached. You can turn it by hand unless the compressor is seized. The front part will rotate (with some resistance) even though the belt is not moving because the clutch is never engaged with the car off. When the clutch IS engaged (and car running), this part is locked to the rest of the pulley via the clutch and would be turning. If the clutch is not engaged this part will remain stationary when the car is running.
Take a good look at this part as you turn it with your hand. Then watch it again when the engine is running and you will know if the compressor is turning or not by watching the front part of the compressor. If it is turning the clutch is engaged.
I would recommend two things. First if you want to work on the a/c, buy a generic a/c manual like I did to get a better understanding of the working pressures, etc. and common faults. Second, check that pressurestat to see if you lost your refrigerant like the previous post recommended. It sounds like you have a voltmeter so put it in ohms mode and take a reading across the two pins that you see when you remove the connector from the receiver-drier. (Don't take a reading on the connector with the wires). If it is shorted you still have some pressure, if it is open your clutch will not run because of a leak. You lost most of the pressure. If that is the case you will have to take it to the shop or spend the $300 or so it takes to get the vacuum pump and pressure gauges.
On the positive side, once you buy that stuff you can fix any problem in the a/c system with some effort.
Go ahead and take a meter reading on the pressurestat and get back to us.
Most cars cycle the a/c to prevent freezing. Listen to someone elses car running with the a/c on. You can hear a distinct slap when the clutch engages on most cars especially if you have the hood open. It may happen every 10 seconds or so.
Good luck with it. I will try to help if I can but please be careful. A belt can take a finger off. Keep your hand clear of them.
R Duke
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