You pick it up,shake it a bit and guess! Wish I knoew as ours has not run out for ages and sods law sells it will when we have lots of people over, and you can't just have a spare one, as they need re-filling!
a pressure gauge on the regulator valve is the surest way. Other than that, check the flame - does it look as if it is switched to low even if the gas is on high? If the pressure is low, that is a good sign that the gas is nearly out.
The pressure is only significant when the tank is almost empty (like about to run out completely), and is dependent on the temperature. That's because the gas is stored as a liquid with a vapour space above it. The pressure in the vapour space is dependent on the temperature of the gas, not on the volume of the liquid.
The best way is to weigh the cylinder and subtract the empty weight. According to Calor's website, the tare weight is printed on each cylinder.