Two ways. Natural gas is odorized and is meant to be sensed as a smell, so it is often obvious. Or, P.G. & E will notify customers if they suspect a leak. Any leak found on the house side of the meter is the property owner’s responsibility to remedy.
Flexible gas piping can save time and money when replacing an existing system. Its much easier to service those “hard to reach” places under existing homes with a flexible pipe. When room and space is sufficient, however, steel can be the best choice as it is less expensive.
A picture says a 1000 words. Here is Barney Huddleson and Bradley Maples. Barney is in his 33rd year here. His skill level is at the pinnacle of any measurable standard for plumbing. He is outstanding at passing along his skill to our younger ones.