I have seen auto mechanics use an unlit propane torch at a very low setting.
Directing it around the gaskets of a running engine, causes a very
noticealbe RPM increase, should a leaking intake system be present. One has
to be aware to keep the propane away from the normal fuel air intake. I
don't know if this system of testing would be proper in an aviation
instance, where the prop wash may interfere. Maybe, feeding the propane
very close to the area to be checked, in small plastic tubing may help.