Obviously the easiest way is to feel the air ducts inside the car. If they don't feel icy cold, the air con is not working properly.
The system is fairly intelligent in that it does not turn on if it knows theres a fault in the system. In ours, one of the pipes is fractured and because the system does not see any pressure, the compressor does not engage.
If you look in front of the engine, you'll see a unit with two large dust covers (like the ones on your wheel valves) pointing upwards. This is where the system is charged. Grab the pipes to the right of this unit to see if they are in one piece or not. I think they have a tendancy to fracture.
