I would like to profer two methods. 1/ We know how fast various electric motors rotate at given voltage/amperages. Simply use a multimeter to measure the amperage being consumed and do the math. 2/ Most fans are occilating. With the power OFF and the oscillating function ON, Manually spin the blades and COUNT how many turns of the fan it takes to complete 1 SWING (1/2 swing would do) of the entire apparatus. Replace everything and time how long when running it takes to complete this same occillation and do the math utilising your findings from the manual test.
Kindest Regards. Geoff C