You can sniff till you're blue in the face and it doesn't tell you if you have a leak.
In order to test a system, you plug all the service connections (water heaters, stove, dryer, etc.) and pump up the system with a compressor.
That should have been done when everything was installed, it's code.
Then when the gas company turns on the gas, they do the same thing.
If you have a real fear of a leak, the LAST thing you should be doing is doing this yourself. If you're jut being paranoid, knock yourself out.
