How to check if a function has an implementation at run time in C++11?

I have only the header file which has the declaration of the functions, but I can't access its corresponding source file. I can change the header file accordingly or I write a function to check if a particular function of a particular class is implemented.

If I made the function in the base class as a pure virtual function, if there isn't an implementation in the derived, then I won't be able to compile it. Instead, I need a checker to verify its implementation at run time.

Could you please help me understand what needs to be done?