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...<br />
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