As we cannot see through LPG cylinders, this can be difficult! The most accurate way is to pour hot (but not boiling) water down the outside of the cylinder and then run your hand down the same line. There will be a change in temperature (colder) when you reach the LPG level as the gas inside is a cool liquid. A condensation line will also form, which will be visible. This will give you an idea of the quantity inside. Check this level against the date you started this cylinder (usually not the date of delivery) to estimate how much longer it will last. Please note that cylinders are never filled to the absolute top to allow for expansion and contraction of the liquid.