1. The frost line. Pour warm water down the side of the cylinder while it's in use. Where the frost forms is the top of the liquid LPG — everything above that line is empty.
2. Flame colour. A healthy LPG flame is blue with a small yellow tip. If yours starts sputtering, going orange or dropping in intensity, pressure is falling.
3. Lift test. A 9kg cylinder weighs ~17.5kg full and ~8.5kg empty. If it feels suspiciously light, it probably is.
4. Cooking time. If boiling the same kettle takes noticeably longer than usual, gas flow is dropping.
5. Use GasCheck. Track your daily burn rate on the calculator or Dashboard and you'll always have a solid days-remaining estimate.