how to find the "true" entropy of std::random_device

I want to check whether my implementation of std::random_device has non-zero entropy (i.e. is non-deterministic), using std::random_device::entropy() function. However, according to cppreference.com

"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."

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?