How to know gas cylinder is empty?
A simple low-tech method my grandmother taught me: Soap water test. Mix some liquid soap with water to make a soapy solution. With the gas turned ON at the burner carefully apply the soap solution along the seam of the cylinder valve. If you see bubbles forming it means gas is escaping (which it shouldn't when closed) but this also gives you an idea of pressure. When the cylinder is full even a tiny leak creates lots of bubbles quickly. When nearly empty the bubbling is much slower if there's any leak at all.
But honestly the most reliable way is to just have a spare cylinder ready. In India most distributors offer a replacement within hours if you call when you notice the flame weakening. Don't wait until it's completely dead - order a replacement when you see the first signs. Safety first: never try to heat the cylinder or tilt it to get "the last bit out" - that's extremely dangerous!
From someone who's been caught mid-cooking with an empty cylinder too many times: The hot water trick works but be careful with the water temperature. I use warm tap water not boiling to avoid any risk. What I do now is keep a backup cylinder always connected via a double-burner regulator. When the first one empties I just switch the knob.
Also pay attention to sound! When you shake a nearly empty cylinder you can sometimes hear the liquid sloshing at the bottom. When it's completely empty it sounds hollow. And if you're using it for a gas geyser or heater the pressure drop will be obvious - the water won't get as hot. My best advice: note the date when you get a new cylinder and track how long it lasts for your household. You'll start to predict when it's about to finish.
There are a few reliable ways to check if your LPG cylinder is empty or running low. The most common method is the hot water test. Carefully pour hot (not boiling) water down the side of the cylinder. Wait a few seconds then run your hand along the metal. The part of the cylinder that still contains gas will feel cool to the touch while the empty part will feel warm because the metal heats up quickly. The line between cool and warm shows your gas level.
Other signs: 1) Weight: A full 14.2 kg domestic cylinder weighs about 29-30 kg total (cylinder + gas). An empty one is around 15-16 kg. If you have a scale you can check. 2) Flame behavior: When the gas is very low the flame becomes weak yellow and might keep going out. 3) Frost line: When gas is being used at a high rate you might see a frost line form on the cylinder showing the liquid level inside. Always be cautious and never use open flames to test! For accurate tracking consider getting a gas level indicator that attaches to your regulator.