Frequency of a rotating machine can be found by Stroboscope..
It makes use of Stroboscopic Effect.When the frequency of
the Incident light is equal to the rotating speed (measured
as frequency of the point moving ) it appears to be
static...and the corresponding speed gives the speed of the
machine..
Or it can also be measured by a tachogenrator...

U can first measure the rotor speed at full load using a 
tachometer..note down the Rated speed frm the panel board 
and take it as NS.. rotor speed as NR.
Slip is  equal to NS-NR/Ns....so the rotor freq Fr is 
slip * AC supply frequency