How to determine programmatically if an expression is rvalue or lvalue in C++?

Most of the work is already done for you by the stdlib, you just need a function wrapper:

template <typename T>
constexpr bool is_lvalue(T&&) {
  return std::is_lvalue_reference<T>{};
}

in the case you pass a std::string lvalue then T will deduce to std::string& or const std::string&, for rvalues it will deduce to std::string

Note that Yakk’s answer will return a different type, which allows for more flexibility and you should read that answer and probably use it instead.