1. Run a query to get you a list of people who have opted out of email.
  2. Use the query to mail merge the list into a .csv file.
  3. Download the .csv file.
  4. Re-save your original spreadsheet as a .csv file.
  5. Open and sort the files by email address.
  6. Use Excel's V LOOKUP formula to see the differences
Example formula:

Position your column, one value per row, in column A of each spreadsheet. in column B of the larger sheet, type
=VLOOKUP(A1,'[Book2.xlsb]SheetName'!$A:$A,1,FALSE) 

Then copy the formula down as far as your column of data runs.

Where the result of the formula is FALSE, that data is not in the other worksheet.


http://office.microsoft.com/en-us/excel-help/vlookup-HP005209335.aspx