Easy, there are a couple if tits on the pipes, one near the compressor and one near the firewall from memory, they look just like the valve caps on your tyres but bigger, you'll find the same type of valve underneath it too, give it a very quick press with a key or screw driver, if the gas is highly pressurised it'll spurt out with a bit of oil, if it's empty it'll be the same as a flat tyre...nil to little pressure.