Mary,
Tough call without a size comparison or eye location. Tail I.D. is a good one too. I usually see the Coopers sitting still for hours in a perch very alert and Sharpies a little less complacent. In my experiences they seem to not sit as long as Coopers but that could just be me as I have never read anything about this behavior. I will send you an e-mail with an attachment of an image of a side by side comparison of a Sharpie and a Cooper's, hopefully this will help.

Bryan
Shoreline
jennandbryan at msn.com