Measuring frequency of a fluorescent light source?
I personally have done this with a Greenlee clamp meter, a volt-meter with a Hz setting. I connected the meter to the terminals at the non-shunted end. Only do this with a Cat II or higher rated meter and only after the light has lit. Otherwise the meter will be subjected to the ignition voltage of near 1kV.

I rather liked the high-frequency ballasts that I installed. Went from 60Hz to 48kHz. These were Philips high frequency electronic ballasts.

I got T12 because I didn't want to replace the bulbs. I really should have at the time got T8 ballasts and replaced the bulbs.

I would not have traded for LED tubes. Most the the LED tubes are poor replacements they tend to be far too blue and also lack diffusers so you end up with point light sources brighter than the sun. Which exceeds the recommended the luminous contrast ratio between light and dark areas in a office work space. The point light sources are so bright that they make the area effectively darker.