The Monitor is located in the bottom right of the synth.

Click the monitor to expand or collapse it.

Change the monitor type by clicking the Arrow Buttons.
There are 2 types of monitors:

Oscilloscope

Shows 2 cycles of the current audio being output.

The audio is post effects etc, so you will see the result of all filters and effects.

As you play, the oscilloscope adjusts to the frequency of the notes being played. If you're holding a chord, the scope may become unstable.

MIDI monitor

The MIDI monitor shows a vertically scrolling history of MIDI notes that have been played.