non-member function overloads

Hi,
  How do I find out all the non-member function overloads for a certain type
in clang. E.g. I have a class string. I want to find out whether this has an
overloaded operator of type "ostream operator<<". How can I do that? I tried
iterating through all the member functions and friend functions but it
appears that the operator is defined as a non-member function overload. How
can I find all non-member function overloads for a certain type (e.g.
string)?

Regards,
Adil