How do I know if my idle air control valve is working?

If it's a modern vehicle, you can use an OBD2 scanner to check your idle air through scan tools for certain makes and models. You can also try to manually activate it to see how the car reacts—if something is malfunctioning, you can see if it's the valve or something outside the valve that's causing the issue. Sometimes the check engine light will go off, which can also help with the diagnosis. If it's an older vehicle, check the resistance that's going to the idle control valve from the vehicle; if it's outside of normal design numbers/parameters, you can isolate the problem accordingly.