Get a digital camera and look at the screen/view finder while pointing the TSU9400 to the lens.
You should clearly see the IR LEDs light up on the camera screen when you send an IR command.

I remember reading somewhere that this would not work with an iPhone as those allegedly filter out the IR range.
I did check with my iPhone 12 just now and I could see the IR LEDs on my TSU9800 light up, though, so this might not be a thing anymore.

Sebastian