When evaluating C++ developer candidates, it is essential to assess their proficiency in core C++ language features such as object-oriented programming, templates, and the Standard Template Library (STL), as well as their knowledge of modern C++ standards like C++11, C++14, and C++17. Strong problem-solving and algorithmic skills are crucial, along with experience in relevant libraries such as Boost, Qt, POCO, and OpenCV, software development principles, and potentially domain-specific knowledge like embedded systems or game development based on project needs.