Like Vasille says in the comment to this question, if your file is not actually in UTF-8 format, you may technically want to convert the characters within the file to the encoding you want before opening in Excel. For some other encoding, use the code page identifier, which you can find in the Text Import Wizard in Excel or in this list. I found my answer at Changing default text import origin type in Excel.
I answered a similar question at Default character encoding for Excel Text Wizard?.
