Measuring frequency of a coin vibration motor
The vibration rate is probably in the low audio range. So here are some ideas:

Connect a microphone to your scope, put it near the motor, and measure the frequency of the waveform.
Use a smartphone to record the buzzing sound, then analyze it with an audio editing app.
Record the sound using a laptop, again analyzing with an audio editing program.
Get an adjustable strobe light, adjust it to “freeze” the vibration and read the frequency from the strobe rep rate.
An accelerometer (analog) and your scope.
An accelerometer (digital) and a microcontroller.