I run a randomness company. It's impossible to prove it 100%.
What you can do instead is run code audits, analyze the theoretical basis of the entropy source, and test large amounts of data for its statistical properties. That can get you to near certainty, but it's still empirical.
Both, depending on the desires of customers, but it's currently leaning towards high-end hardware/software. A broad-market randomness-as-a-service offering is coming this month.
What you can do instead is run code audits, analyze the theoretical basis of the entropy source, and test large amounts of data for its statistical properties. That can get you to near certainty, but it's still empirical.