Other than listening, I believe your only other option is checking the documentation to see if it is mentioned/supported. Not all devices support velocity variations.

The manual for the SAM2695 has a midi implementation chart which shows that it at least accepts all velocity values, but I don't see any mention of it actually responding to them.

If you want to get technical, you could record the sound directly in to a computer without actually listening to it and view the waveform to see if there is any variation with the change in velocity. Perhaps forum user Sema can provide a test file with alternating velocities that you could play back to test it out? Assuming that you have a way of sending a mid file to the device.

Thanks for mentioning me!
I've added a test named test-note-on-velocity.mid at https://github.com/jazz-soft/test-midi-files
Most likely, organ will ignore velocity.

Quoted from Christoph on Saturday, 22 April 2023

From my understanding many midi voices react upon the velocity parameter in the "note on" command. With a low velocity value (vv) a piano, e.g., sounds mellow or smooth while giving it a vv of, say 127, it may sound harsh or strong, just as a piano sounds when you hit the keys hard.

How can I tell whether a midi voice reacts upon vv (other than actually listening to it)? I'm considering giving my organ bass pedal unit constructed from a SAM2695 and a little STM32F103C8T6 (blue pill) microcontroller an extra potby which I can influence the sound of, e.g. the voice "acoustic bass" a different flavor soundwise.

If you're obtaining midi voicings from a reputable source the details of the velocity will be in the user manual. Otherwise you will have to manually adjust everything on your own which isn't so hard if you have a MIDI editor or DAW that allows you to make broad adjustments to numerous midi notes at once, i.e. Cakewalk by Bandlab.