Carefully spray carb cleaner over the air inlet track. If the revs go up you have a leak.
Do it out side and carefully unless you want an explosion.
Theoretically that would work but an aerosol will mean that the air cleaner inlet will get a sniff of it so not proving anything. This was demonstrated on my Land Rover, albeit diesel, after cleaning up with brake cleaner following fuel lift pump renewal. I was surprised how much the idle speed rose.
