If you are refering to your intake manifold gasket. With a cold engine, spray the gasket/im/head area down with wd-40. If the idle changes, your gasket is leaking.
If you are refering to your exhaust manifold gasket, you should see the exhaust "puff" out around the exahust manifold when cold. You should see black carbon deposits around the leak. When the engine is cold, hold a smoking cigarette, or stick of insense up to the exhaust manifold. If you see the smoke get distrubed by fast moving air...you know you are leaking.
The incense stick idea also works on the intake manifold, but you do not want anything HOT there, as if air is escaping from the IM/Head gasket....that is air and fuel mixed together...which means the incense/cigarette could ignite the mixture in your face!
Leave it to Random to Needlessly complicate things.
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