The shake method
The simplest way to tell how much gas is left in your bottle is to disconnect it and shake it around to get the liquid to “swish” back and forth and you will get an idea of how much gas is in the bottle. If the bottle is heavy to move around its most probably quite full, If the bottle is easy to move it may be near empty or empty.
The hot water method
A more accurate method is to pour hot water down the side of the gas bottle and then run your hand down the bottle (where you poured the hot water) and the steal of the bottle will be warm until you reach the level of the liquid in the bottle. The place where the steel becomes cool or cold is the level of liquid on the bottle.