How to determine if va_list is empty

I have been reading that some compilers support va_list with macros and users were able to overload the functionality with other macros in order to count the va_list.

With visual studio, is there a way to determine if the va_list is empty (aka count==0)? Basically I would like to know this condition:

extern void Foo(const char* psz, ...);
void Test()
{
  Foo("My String"); // No params were passed
}

My initial thought was to do something like this:

va_list vaStart;
va_list vaEnd;
va_start(vaStart, psz);
va_end(vaEnd);
if (vaStart == vaEnd) ...

The problem is that va_end only sets the param to null.

#define _crt_va_start(ap,v)  ( ap = (va_list)_ADDRESSOF(v) + _INTSIZEOF(v) )
#define _crt_va_arg(ap,t)    ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) )
#define _crt_va_end(ap)      ( ap = (va_list)0 )

I was thinking of maybe incorporating a terminator but I would want it to be hidden from the caller so that existing code doesnt need to be changed.