Just recently by pure accident I found out that it's possible to see the shadow of the veins on your retina.
If you want to try this yourself you can follow the instructions here:
https://www.aao.org/museum-education-healthy-vision/experiment-see-blood-vessels-in-your-eyeAlternatively, you can try the same way I did it:
Stare at your blank TV
With your arm fully extended, move your arm about a foot from center and hold a light source (I used my phone) and point it in the same direction as your eye is looking (towards your eye)
You can move your arm around in a circular fashion to see different regions within your eye
As mentioned in the article above, you can even use this method to identify blind spots such as the optic disk which is where the veins come into the eye from. I suppose if you have any retinal degeneration you could also identify that.