For Excel 2010 it should be UTF-8. Instruction by MS :
http://msdn.microsoft.com/en-us/library/bb507946:
"The basic document structure of a SpreadsheetML document consists of the Sheets and Sheet elements, which reference the worksheets in the Workbook. A separate XML file is created for each Worksheet. For example, the SpreadsheetML for a workbook that has two worksheets name MySheet1 and MySheet2 is located in the Workbook.xml file and is shown in the following code example.
The worksheet XML files contain one or more block level elements such as SheetData. sheetData represents the cell table and contains one or more Row elements. A row contains one or more Cell elements. Each cell contains a CellValue element that represents the value of the cell. For example, the SpreadsheetML for the first worksheet in a workbook, that only has the value 100 in cell A1, is located in the Sheet1.xml file and is shown in the following code example.
100
"
Detection of cell encodings:
https://metacpan.org/pod/Spreadsheet::ParseExcel::Cell
http://forums.asp.net/t/1608228.aspx/1
