Inspectors should use a soapy water solution or a commercially available, non-corrosive leak detection fluid to find leaks in gas piping. If leak detection fluids are not approved as non-corrosive, rinse the pipe after testing.
Electronic leak detectors are also designed to detect the presence of combustible gas. Most of these types of leak detectors will measure in parts per million. They need to be calibrated from time to time. Care must be used because they can detect minute leaks that are less than that allowed by code. Electronic leak detectors are typically more high-tech than inspectors need.
This said, most of us just use our nose!