If you see a red X on your map, you have found one.

There is no special way of finding them without just getting a list from the spoiler forum.

I dont think I am spoiling to say that you will find them in out of the way places that other people are unlikely to visit. Normally in Asteroid fields or Nebulas of remote systems.

or places filled with radiation...

Why the heck can't your shields absorb this? You would think someone would design shields to block radiation sooner or later and make billions selling it to the houses.