There is eCOGRA Commission which is one way to prove that Random Number Generator generates pseudorandom numbers or uses external source or randomness also  approved in online casinos.

And question about Hardware RNGs (HRNG),

I'll dedicate more time to research this but please tell me where I can find if online casino uses external source of randomness ( Hardware random number generator ) ?

These are HRNGS :  Quantis, Quantis + MT
   and  ComScire

Which property of RNGenerator is eliminated during code revision (in case that RNG it is software based generator ) ?
Hardware generators are probably bought with some anti-tampering protection and with certificate from the manufacturer

  I guess they inspect code and certify somehow , I assume they use digital signature and after that Certification authority provides certificate that RNG is according to standards ?
I hope I am right    ?