Nearly there, Hall effect square waveform pattern? Crankshaft angle sensor which doubles up on a lot of engines as a speed sensor, read rpm straight from the horses mouth. Camshaft angle sensor which obviously runs at half the crank rpm, your pulse theory nearly there, times the result by two to get crank rpm. Or older smaller engines off with flywheel inspection cover, white chalk TDC marks and strobe rpm using a stopwatch. Or get really with it on a slow speed job, read reduction label on gearbox work out prop. RPM again with strobe light or count a whitened coupling bolt going by to get rpm crank turning at. Nearly there? Or a mark two futuristic drone hovering in the crankcase strobing the crank rpm for you and remotely reporting back to you in the bar. Who knows what's next in this technology world of today? Oh, and in passing if you are running a two stroke don't look yourself for the Camshaft angle sensor, send the Cadet/ apprentice instead. Happy new year and best of luck with your new rpm hobby , different I have to say. No gold watch for you apon retirement just a portable handheld rev counter methinks, imperial of course.
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.