If you've already mastered c++ itself, you can try to overview standard library. Also, depending on what you want to do in future, you might want to learn gui - cross-platform library or specific API, graphics for games, algorithms.
Ok, but what concepts do I need to understand that make me an expert in c++?
In other words, what are some indications that I am getting close to being a c++ expert?
(Im not talking about any libraries, just the raw language)
If you want to know language very well I think you should know classes, inheritance, polymorphism, pure virtual functions, abstract classes, functions, overloading functions, templates, variadic templates, meta-programming would be nice, pointers, references, pointers to members, flow control statements, enums, unions, uniform initialization, everything about constructors exceptions, fundamental types, arrays, scopes, c-strings, and probably way more that I can't think of right now.