You can still get tach generators. I just used one on a recent project as feedback on a shunt-wound DC motor. They're getting harder to find now though.
A resolver is also an option if you want to get fancy.
In the good old days, mid seventies, I use to just use a magnet attached to a rotating shaft and a loop of wire. Connect to a volt meter and wait for the needle to deflect. Counting the spikes in a minute gives you RPM. Same principle applies today, just fancier equipment.
Good luck at high speeds!
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada