You could use a digital multimeter that can measure frequency(Hz) to find what frequency your zapper is running at. Most entry level meters won't be highly accurate, but will usually come close enough to give you an good idea of what's going on. Whereas some of higher-end meters can measure with much higher accuracy and even provide data capture /means. ie, up-to 200kHz ±(0,02% + 4 digits) type thing.