Lots of inexpensive options. The $1 option is to just label your potentiometer -- especially if it's a decent VFD with some sort of vector control so speeds don't change much under load.
Your VFD potentiometer probably has (or could have) tick marks from something like 20% to 150% (or whatever you think your motor and bearings can handle) of nominal speed. By measuring the speeds with a hand held tach one time, you can make up a little laminated chart for each VFD and pulley setting.
One advantage of a chart - even if you decice to add a tach -- is that you will actually know the options of where to set both the VFD and the pulleys to get the speed you want. I Just the pot setting won't be accurate to the rpm (speed will vary a bit depending upon load, line voltage, etc.), but plenty close enough to set initial speeds and feeds. From there, you just listen to the cut and watch the chips to turn the speed up or down a bit.
My vertical bandsaw is set up with tick marks at 50, 75, 100, 150, 200 fpm. A small gear head lathe has the pot calibrated for half, nominal, and 2x speed. A 20" step pulley drill press has a chart on the side with a speed range for each pulley setting. With a VFD, I usually can stick to a single pulley setting, but know instantly what to change for very large or very small drills. Only a mid size lathe and a wood lathe seemed to want/need an actual tach. Nice if you can put the display right with the VFD control.
