The places I fuel up are too noisy to listen for audio clues.
Here's what I do: My vent splash cap will unscrew, leaving a plastic nub
that I shove a short length of stiff clear hose onto which points up. When
the fuel comes up the vent line it will bubble into the tube where I can see
it. But see below.
Here's another tip: While you are pumping fuel there should be a steady
stream of visible gas fumes coming out of the vent. As soon as the bottom
of the vent line is full of fuel the fumes will diminish greatly, meaning
that the tank is full(enough). Any furthur fueling will probably fill up
the filler tube as well as the vent line, and if the gas expands, or if any
trapped air in the tank warms up it can push some fuel out of the tank and
cause a spill. Sometimes a real full tank will spill some during
trailering, at least on my boat.
On many boats the routing of the vent line has one or more dips in it as it
rises to the overflow outlet. If you have overfilled the tank these dips
will trap liquid fuel and prevent the fumes from escaping during normal
fueling operations, and can contribute to the pump handle shutting off
early, or spitting back, as the fumes try to come back up the filler
tube/neck. If possible, route the vent line, or support it to eliminate any
dips that will hold fuel. If I don't see fumes coming out of my vent while
fueling, then I assume that there is some fuel trapped in a dip in the vent
line, so I blow(carefully) into the vent tube to clear the line before
continuing to fuel up. This is probably not an advisable procedure, but it
works for me, and I don't like fuel spilling out onto my swim
platform/boarding ladder well. I just ignore the gas station attendant
coming over the speaker: "pump 6.....welcome to Hess.....what are you
doing?"