The compressor will not engage if the pressure is too low. Checking the pressures with a set of gauges is the sure fire way to find out.

If you do not have a set of gauges, there is a simple test to be sure if you have the voltage needed and if the pressure is to low. Take a volt meter and find the low pressure switch on the dryer. It's located over on the passengers front side of the car. The switch is right on top of the dryer and they are green and green/white I think. Start the car and turn the AC knob fully CW. Go to the dryer switch and touch the negative lead of the meter to the chassis and touch both sides of the switch contacts with the plus lead. If one side of the switch has 12 Volts (green I think) and the other does not. If only one side has voltage your system pressure is too low and that is why the system will not kick on. Now if you take a couple of spade contacts and a little wire, you can use that to see if the compressor kicks on. Just use the jumper between the to wires on the low pressure switch. That should send 12 volts to the compressor. A few seconds after you start the car you should hear a click and the compressor should start rumbling. Do not run the car that way, there is a reason for the low pressure switch.

That will also tells you if the Relay and voltages inside the car are working. If you don't have 12 volts on at least one side of the switch, I will start by checking fuse 12, the fuse panel is on the drivers side of the car under a plastic cover.

Good Luck,

Paul