Soapy water is what's for bubble testing LPG, NG etc... But if you would like to purchase something specifically made for this test purchase a product called Snoop for around $9.00. Every fire needs an ignition source or spark along with fuel and oxygen to combust. Have you ever seen under the hood of a forklift car etc... Especially when it's dark on a vehicle that's in need of a basic tune-up? It looks like lightening during a thunder storm and in your case case when you have a fuel leak it will catch fire and possibly explode if the leak is severe enough.