Show cells have been read - linked
 

I have several (10 different companies) workbooks reading (linked through
"SUM") a sales forcast from another workbook called SALES.
I have highlighted the linked cells in the 10 individual workbooks to show
which cells are linked and update automatically.
BUT... now I need to see which cells in the SALES workbook have not been
read into the individual ones so I can check which cells have not been read
yet.
How would I go about differenciating between cells that have or have not
been read?
Is there a conditional format or a formula that would work?
Thanks in advance for any help you can offer