Fuel leaks can be observed in a number of ways:
Visual
If a leak exists in the high pressure side of the system there may be oil residue or ice present at the point of the leak.
Audible
Depending on the size of the leak you may hear sounds ranging from a slight hissing to the sound of rushing air.
Smell
CNG has a distinctive odor. The fuel in your CNG cylinder is the same fuel and has the same odor as the gas from your natural gas stove at home.
For hydrogen cylinders the fuel is not odorized and you will not be able to detect a fuel leak by smell. Consult the vehicle’s manual for more details.