Hows about this? It is just an idea, so you will need to test yourself.

Once you are about to do the save, try opening and closing a dummy file with
the EXACT same path and filename, but use the function FILE_OPEN_WRITE().
If it SUCESSFULLY opens (ie returns NOT zero), then you have the rights.
If it FAILS (ie returns a zero), then you are unable to save here(not admin?).

Im just not sure if this will trigger a windows error/warning if you are not admin.

Also beware, you may need to delete the dummy file(file_delete) before you actually save.
And when doing this make DOUBLY sure you give file_open_write the COMPLETE file-path.