Knowing how much gas is in a cylinder helps you know when you need to order more. If you have no idea how much gas is left, you have no idea when it will run out -- and that means it's likely to run dry in the middle of a job. Fortunately, gas suppliers realise this and provide their customers with a couple of ways to check the amount of gas left in a cylinder.
Check to see if the cylinder has a gauge that tells you how much gas is left. Open the connected valve and read the gauge if this is the case.
Look for a tare weight on the collar of the cylinder if it doesn't have a gauge. The tare weight -- sometimes written as "T.W." -- is the weight of the cylinder when it's empty.
Read the cylinder's label to determine the weight of the cylinder when it's full. Subtract the tare weight from the filled weight to calculate how much gas is in the container when it's full.
Weigh the cylinder on a shipping or industrial scale. Subtract the tare weight from this measured value to determine how much gas is in the cylinder currently.
Compare the weight of the amount of gas currently in the cylinder against the weight of the gas in a new cylinder to determine how much gas you've used and what is left.