Being a trumpet player for nearly 40 years, I love taching by ear.. you can get very close indeed. It is a good way to weed out BS at the field or on Youtube videos, for example KMD 2.5 diesel team race engines turning 8-4 props at 24k and the like.
I have an Excel chart I keep posting somewhere. It's easy - A above middle C is 440 Hz = 440*60 = 26,400 rpm. Multiply by the 12th root of 2 to go one semitone higher, divide to go one step lower, and just build a table from A=440 down. (or up too if you need).