How can I see if my filterless class-D audio amplifier is clipping or not?
In the end, what you want to know is if the movement of the loudspeakers membrane is according to the voltage waveform you put into your amplifier.

While you surely could do that with a mirror, a laser and some detector, the intended product of the membranes movement is of good use here too: The sound.

Since your goal isn't to verify a flat bode plot or anything, and you just want to see if there is any clipping or other distortion, all you need to do is basically some THD measurement.

Start with a low amplitude 1kHz waveform, record it with a somewhat nice microphone and audio equipment (no need to be too fancy) and put it into whatever software you like to observe the FFT. The harmonics you see there should be your baseline in THD (if your software is fancy enough it will take the harmonics and calculate a single THD figure for you).

If you want you can do the same for a well known good pure 1kHz audio source as a baseline so you know how good or bad your amplifier is already at small amplitudes.

Now increase the amplitude until you see the ratio between the frequency of interest and the harmonics getting worse, i.e. harmonics start to get higher. This is the point where you start getting distortion which is a bit before what people would call is clipping.