> What is the most trivial way to test it's behaviour?

With CREATE_NO_WINDOW, the child process is attached to a console that has no window. Thus calling GetConsoleWindow() in the child returns NULL without an error. OTOH, if the child has no console (e.g. DETACHED_PROCESS or executing "pythonw.exe"), then GetConsoleWindow() returns NULL with the last error set to ERROR_INVALID_HANDLE (6). For example:

    script = r'''
    import sys
    import ctypes
    kernel32 = ctypes.WinDLL('kernel32', use_last_error=True)
    kernel32.GetConsoleWindow.restype = ctypes.c_void_p
    ctypes.set_last_error(0)
    result = kernel32.GetConsoleWindow()
    status = bool(result or ctypes.get_last_error())
    sys.exit(status)
    '''
    args = [sys.executable, '-c', script]

The child's exit status will be 0 if CREATE_NO_WINDOW works as expected. Otherwise the exit status will be 1.