Poor man's tachometer: On a machine that is supposed to put out 60Hz aux power, an old fashoned clock with a sweep second hand works just as well. At 2100 RPM it would LOOK fast. When You Compare to a watch, You know for sure. These old clocks rely on the 60 Hz to keep time. The power company holds really tight tolerances on the frequency.
I have an old Hobart Power/Weld that welds at 2200 RPM but puts out 60Hz at 1800. However in an attempt to be able to use the aux power while at welding speed, the aux. voltage is a compromise between the different RPMs. To maintain enough voltage under load to properly run heavier loads, like the well water pump I have to increase the RPMs to 2100, which gives 70 Hz. Colcks just look funny when they run a bit fast.