Depends upon your definition of small. Is there an exposed shaft? What is the diameter?
Link to a variety of small hand held tachometers and tach sensors:
I've got a mechanical rotation counter in my toolbox. It is made by Starrett. It comes with several different sized small rubber couplings that you hold against the shaft. Run it for a fixed amount of time and do the division and you get the speed. You can probably find one at a tool dealer.
Your motor is probably fastened onto a drive.Perhaps it would be easier to measure the RPM somewhere on the drive. With a tinny color strip on a rotating part of the drive you can measure rotational frequency and calculate the RPM of the motor.
m777182
You can always put an accelerometer on it or nearby, and find the rotational frequency.
If it's not clear from that, unbalance it slightly, then try again!
If it's a commutated motor, we have two products that measure motor speed from commutation current. The first product is older technology, model CRSM1, but we have a new, not-yet-announced product that can track motor speed well enough to allow it to do time-domain averaging of vibration caused by the motor's imbalance.... Contact me for more info if your motor is commutated.... eraftery@cdasys.com