C / C++ / MFC

template <typename _CountofType, size_t _SizeOfArray>
char (*__countof_helper(UNALIGNED _CountofType (&_Array)[_SizeOfArray]))[_SizeOfArray];
#define _countof(_Array) sizeof(*__countof_helper(_Array))

class MyClass {
    friend ostream& operator <<(ostream& stream, const MyClass& obj);
    friend istream& operator >>(istream& stream,       MyClass& obj);
}
ostream& operator <<(ostream& stream, const MyClass& obj) {
    // How do I do the following?
    if ([stream is opened for binary writes]) {
        ...
    } else {
        ...
    }
}
istream& operator >>(istream& stream,       MyClass& obj) {
    // How do I do the following?
    if ([stream is opened for binary reads]) {
        ...
    } else {
        ...
    }
}