When suspecting air leaks in an engine I tended to use a length of flexible pipe, one end to my ear and working the other end around the suspect joints listening out for the hiss to identify the failed inlet gasket. I have used the same method balance the throttles on multiple carb engines, by adjusting the throttles to provide the same degree of hiss, have checked the accuracy of the hiss method by using a flow meter to prove it was a worthwhile method, which it was...
As for the length of pipe I suspect perhaps 3ft or 1meter in length and anything from a garden hose down to circa 3mm bore would work just fine...
I have known of folk spraying Easy Start or similar around a running engine where there was suspected leakage of inlet manifolds and listening for engine rev increases but that seems a tad too dangerous for me ..?
It would seem a pain to strip things down looking for a leaking gasket and finding no gasket issues, other than those created by striping things down..?
Best wished in resolving any such issues...
