Well, here are the first lines of my <windows.h> header: (I'm using Visual Studio 2003, but that shouldn't matter, as Microsoft = backwards compatibility :-))
Code:
#ifndef _WINDOWS_
#define _WINDOWS_
#ifndef WINVER
#define WINVER 0x0501
#else
#if defined(_WIN32_WINNT) && (WINVER < 0x0400) && (_WIN32_WINNT > 0x0400)
#error WINVER setting conflicts with _WIN32_WINNT setting
#endif
#endif
#ifndef _INC_WINDOWS
#define _INC_WINDOWS
So, as I see, you can #ifdef-test either of the _WINDOWS_, WINVER or _INC_WINDOWS definitions.