The old method was to put on a prop a pitch less than the in-flight prop. For example a .60 engine, 11x6 instead of a 11x7 prop. You then tune the engine and pipe to run on the ground OK with the 11x6 prop. The engine unloads in the air, so the 11x6 prop simulates the unloading in the air to some extent.
Then with the 11x7 prop on, you fly the plane and as Bax mentioned you have to listen closely to determine if it needs some more tweaking or not.
But that would get you close for the fine tuning then.
In the past it was popular to hook up a AUX channel on the RC system with a servo connected to the main needle, so you could tweak the needle a little in the air. You could get a 1/4 to 1/2 turn on the needle that way, But usually all you needed was one or two clicks on the needle, so the servo was quite adequate for this purpose then.
As mentioned a Tachometer is really important for this too.