Decades ago my dad had an RPM indicator that had a rubber tip on it;
you pressed the tip against the center of the rotating shaft and it
would read the RPMs.

I've no idea where he got it or what happened to it, or even why he
had it!

David Dewey

 [ Ref. https://blog.qrfs.com/63-which-hand-held-digital-tachometer-is-right-for-me/