Call the gas company.
with matches :)
Smell. In most countries, by law gas has to have a tracer scent because of course you can't just light a match to find out, or see it very easily without the right angles and lighting. Nearly all gas will smell like sulfure; rotten eggs.
I have seen people use a water solution in a spray they when they spray over a pipe that is leaking will form bubbles to show where the leak is coming from.
Soap and water mix...
But if it smells like the sulphur smell they add? open all the windows, get out of the house, call the gas people.
Listen to this one... he/she's know what he/she's talking about.
Dont risk it, get someone out to it