What instrument measures hertz?

Answer 1

An electronic frequency counter, spectrum analyzer, or an oscilloscope, among others.

A much older method is by the use of vibrating reeds!

Meters were constructed using thin metal reeds, which were resonant at particular frequencies. For example, a meter used to measure 60 Hz. could have five reeds mounted in a horizontal plane. The reeds would be calibrated to vibrate at 58, 59, 60, 61, 62 Hz. When you would look at the face of a meter connected to house current (in the U.S., for instance) you could tell whether the frequency was high, low, or right on, by seeing which reed looked the "thickest", since they would vibrate vertically. They were so accurate, that you could even tell if the frequency was 59 1/2 Hz., for example!

Answer 2

Set your oscilloscope for the voltage and read in cycles per second applying a prefix if needed. Hertz, kilohertz, Megahertz, Gigahertz. Well I doubt it can handle the last two.