Fair question .

Ive used propane listening for a rise in idle , and even watched
the tach from timing lite .
Ive yet to do that with a hot hot engine .

Ive Sprayed bottle water to see if it was drawing in.

Other than that , are there better methods . ?

I paid a lot of attention to the outboard pod
Connection to bodies .

Just because , ive done the suck test on the outboards tee hose.
They do open in unison with kill bleeds plugged or not .
This is irrelevant kind of , because i know they open.

I think ive been thorough or im missing a better
More accurate method .