There have been several systems aimed at giving the user an easy means of determining the state of the bottle.
One of the simplest, is strip of temperature sensitive liquid crystal. It is supposed to see a temperature difference between the liquified gas, and the vapour above it. This is based on the way the liquified gas uses the temperature of the liquid to boil the gas to produce vapour. The more gas being used cools the liquid portion, and the liquid crystal will change colour, showing the level of the liquid. If only small amount is being used, the temperature difference between the liquid phase and the vapour may not be enough to show a clear temperature difference. If no gas has been used for an hour or so the temperature will equalise so the indicator will not show the level.
Bottle pressure gauges. As has already been stated, these only really show when the bottle is near the end. This is because whilst there is any liquified gas in the bottle, the liquid will boil to replenish the pressure which is dependant on the temperature of the bottle. Only when the liquid has all boiler leaving just vapour, the pressure will drop as the vapour is used up.
Several attempts have been made to assess the depth of the liquified gas using ultrasonics. Using a transducer fitted to the underside of the cylinder, sonar pulse is emitted, and it's transmitted through the cylinder wall and into the liquid. When pulse reaches a change of density, some of the signal is reflected back. The time it takes to return to the transducer is proportional to the depth of the liquid. These systems can be fooled by reflections from the walls of the cylinder, so they need to be carefully set up.
LPG is almost always sold by weight. This is because the energy it contains is constant amount by weight. So the only way of accurately establishing the amount of gas in a cylinder is by weighing it, and subtracting the tare weight of the bottle. LPG bottles from reputable companies will always have their tare weight visible on the outside of the bottle.