There are sniffers that you can buy that will detect hydrocarbons.
Good for finding propane leaks, and gas leaks.  I bought mine from
Fox Tool & Supply (which used to sell Duracool and now sells a
similar refrigerant product). $150. Helped me fine a leak in my
propane system on my camper, and a gas leak on my passenger vanagon.

<http://www.foxtoolsupply.com/cart/parts.htm>