Shaking the gas cylinder to know the level of its content can be extremely dangerous but probably we all check the level of gas in the gas cylinder by shaking it because this the easiest and well known way of checking.
There is a easy & safe way to check this out. Just wet a side of the cylinder with water. After a while, the used top part will be dry and the lower part with gas will remain wet. The wet portion tells you the amount of gas left.
Shake gas cylinders to check if it is finished or not is really very dangerous & could be suicidal.