I don't always use cutting fluid, but it usually seems to help, and when it starts smoking it's a good indication of excessive speed or dull tool or other problem.
There may be some alternative ways to measure the RPM. If it's a DC motor, the speed is generally proportional to voltage, so a simple voltmeter might tell you the voltage at the known full speed, and you can figure it out from there, or you might even be able to scale an analog (or digital) panel meter to read directly. For instance, a 0-30VDC meter can be scaled with a resistor so it reads 25 at 2500 RPM with an applied voltage of, say, 160 VDC.
If you can get a pulley that is 10 times the diameter of the tool (or the holder), you might be able to put a belt on it and count the revolutions of the pulley over a 1 minute time. This should work up to about 1200 RPM where the wheel will turn 2 times per second which is still slow enough to count. Even a 6 second (1/10 minute) time will work at the higher end of speeds.
If you have a DC permanent magnet motor it might produce a DC voltage proportional to RPM as a generator.
It might not be very accurate to use the sound as a measure of RPM, because it may reflect the number of flutes on the cutter, or you may hear noise from the motor or the pulleys. But with all things being equal, it might work.
If you have an old automotive tachometer it might work if you rig up a simple cam and points with a battery and read the RPM. It will probably need to be scaled because most engines are 4, 6, and 8 cylinder.
And maybe an old automobile speedometer or mechanical tach could be found at a junkyard and used.
Once you have a way to measure the RPM, if the control is indeed linear, you could add markings at several points with the measured RPM.
