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

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.