Propane leaks must be detected by physical signs, since odor alone isn’t reliable. Propane is colorless and the odorant can fail, so you can’t depend on smelling something to know there’s a leak. Listening for a hiss isn’t consistent either, as many leaks are small or occur where you can’t hear them. A practical and reliable method is to apply a soap-and-water solution to joints, connections, and hoses. If there’s a leak, escaping gas will cause bubbles at the leak points, making it visible. If you see bubbles, shut off the gas at the main valve or cylinder, move away, avoid sparks or flames, and call emergency services or your gas supplier. The cylinder gauge shows pressure or how full the cylinder is, not whether there’s a leak, so it’s not a leak-detection method.