The Phanotron will produce the one fundamental frequency that is being used and what you see on Out 1.
There will be sidebands created that are a result of modulation. Simply put, take the fundamental frequency and add or subtract from it the modulation value.
For instance, if I had frequency 1000000 Hz on Out 1, and I use a modulation of 1000 Hz, I will also get 1001000, 1002000, 1003000 Hz and so on up, and 909000, 908000, 907000 Hz going down.
Since the plasma is driven by a square wave, you will also get odd harmonics, where F1 is fundamental, so F3, F5, F7, etc…
As for testing it, grab your phone, grab an audio spectrograph like spectro-gram, and put on a program with say a fundamental frequency of 100000 Hz, and a modulation of 5000 Hz.
You should be able to see spikes in the audio range at 5000, 10000, and 15000 (since your phone can only really pay attention up to 20000 Hz).
It’s a cheap way to confirm if you don’t have higher grade equipment.
Remember, that for each harmonic, the further away from the fundamental it will have less power. This also goes for each sideband generated.