intake gasket may be leaking

i have a 2000 isuzu rodeo and the other day i noticed that the engine was idling louder than usual when in Drive.i did the cheapest thing I could think of which was change the spark plug.however i called around and most of the dealership recommended the denso plug which was outrageous for a spark plug.i decided to purchase it anyway because i'm hoping to keep this vehicle for a long time.while changing the plug, noticed that one of the plug was wet and another one was rusted.with the new plug in, the engine seems to idle fine and there is no heavy vibration but i'm worry about the gasket leaking.what can i do to test and see if the gasket is leaking.

listen for a hissing noise usually more audible when hot or spray intake cleaner around intake while at idle listen for a change in the engine