If you had a guitar or other musical tuner you could have it play the B 2 octaves below middle C which is about a hz or two above 60hz.
Or if you are like me who has worked as a commercial
electrician around transformers and other equipment
for decades, all humming at 60hz, and also did a few installations for medical equipment
manufacturers like GE for diagnostic/testing of equipment being sent to countries that use 50hz. so I am pretty good at knowing what each "should" sound like by ear.
Or if you had an old fashioned mechanical electric
120v plug-in clock you could do it the way old-school generator guys did it back in the day and used the sweep of the second hand matched to a battery-operated clock or wound watch side-by-side. When the second hands are in synch you are pretty damn close to 60hz. If you watched for a few minutes you could get nearly exact.
The issue is every time the electrical
load changed it would change the RPM of the engine
since it would bog down a bit at higher loads and run faster when lightly loaded.