There are several ways to detect a gas leak in a tank or cylinder:
Smell. This is why the odorant is placed in the product. If you smell the unmistakable odor, there could be a gas leak or open fitting somewhere in your propane system.
Oily film around the valve or fitting connections. Small gas leaks often leave an oily residue at the point of leakage.
Sound. A more severe gas leak will sometimes emit a sound from a low “psssss” to a high pitch squeal.
Fuel level decreasing faster than normal. If the level of propane is dropping faster than normal, one possible scenario could be that a gas leak exists somewhere on the container, it’s lines or the apparatus that is being used.
Leaks in liquid space will usually cause a frosting or freezing around the leaking spot.