You can try to shoot it with the uniform color, either toned down white, or one of the RGB set (this will only prove a dead subpixel, but you can have at least something to show).
The other possibility (if you are desperate) is to remove the lens (I have never done it, but some are saying that it is not that difficult @sjefdeklerk @SweViver) and just use a macro lens or a digital microscope.
Shooting through the fresnel lens can be quite frustrating.
Concerning the macro, what I found surprisingly good for macro shots (not of DSLR quality, but good nonetheless) was the “macro lens clip” (which I originally bought for RPi camera - https://shop.pimoroni.com/products/3-in-1-camera-lens-set).
If you have a phone with relatively good sensor (I have Mate 10) and the lens close enough to the body edge so you can fit the clip over the lens entirely (which I cannot on mine :(), then it might be worth a try. But then you have to fit the phone into the headset (or remove the face supporting frame, and eventually remove the lenses).