title: how to find the "true" entropy of std::random_device?
body: <p>I want to check whether my implementation of <code>std::random_device</code>
  has non-zero entropy (i.e. is non-deterministic), using <code>std::random_device::entropy()</code> function. However, according
  to <a href="http://en.cppreference.com/w/cpp/numeric/random/random_device/entropy">cppreference.com</a></p>
  
  <blockquote>
    <p>"This function is not fully implemented in some standard libraries. 
    For example, gcc and clang always return zero even though  the device
    is non-deterministic. In comparison, Visual C++ always  returns 32,
    and boost.random returns 10."</p>
  </blockquote>
  
  <p>Is there any way of finding the real entropy? In particular, do modern
  computers (MacBook Pro/iMac etc) have a non-deterministic source or randomness, like e.g. using heat dissipation monitors?</p>