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Since you don't know how many windings are in your brushed motor, a scope isn't great help so you'll have to add something to the shaft like a rotary encoder. Or even a piece of white tape with a wide black line on it (parallel with shaft. Then illuminate the tape with IR and use a photo resistor to see the voltage drop each time the black line passes.
Resolution: 0.1 RPM (rev/min) (2.5~999.9 RPM); 1 RPM (r/min) (above 1000 RPM). Test Range: 2.5~99,999RPM (r/min). Display: 5 bit 18mm height LCD screen. Range selection: Automatic switching. We will help you step by step for the exchang process.
I've had a similar one for several years. Mine came with a roll of retroreflective tape as well, you just put a small tag of that on the thing you are measuring, if it's not got enough contrast with white paper or normal tape.
Make yourself a 60 segment optical disk and put photo-interrupter across it. Then you can use your frequency counter in Hz to read the speed directly in RPM from the photo-interrupter output (ie 1 rev per second = 60rpm)
(I did this to test a lot of small motors recently, worked very well)