When I was testing to see if the readout on my VFD was lined up with what I thought what RPM it indicated, I used an optical tachometer. I think mine was around $30 off of Amazon. I stuck a little piece of reflective white sign vinyl on the spindle nut, shut the fluorescent lights off, shined an LED flashlight on it, and pointed the tach at it. It worked for me.
Aspire 9.5 - FlexiEXPERT 11 - Vytek Rebel 4' x 8' - 5HP Perske Spindle