AFAIK, it's no possible in old versions of Excel, not sure about 2007.

All programs should recognize unicode files by reading the first two or three bytes, it's standard. e.g. The first two bytes of UTF-16 file are 'FF FE'; the first 3 bytes of UTF-8 file are 'EF BB BF'. You can check it with VC++ or any text editor which supports HEX mode.