how do systems know that their RNG is working. This turns out to be a question without a perfect answer.
From Matthew Green in “How do you know if an RNG is working?“.
So much of electronic security comes from cryptography that depends on good random number generators ( RNGs ) and the pseudo random number generators ( PRNGs ) usually built on top of them that a compromise at that level becomes a very big issue.
Which brings up the question of testing RNGs. Go read the post by Matthew Green, it turns out to be a very difficult problem.