See Excel 2007 document path in title bar

http://exceliseasy.wordpress.com/2012/10/29/show-file-path-in-excel/
http://www.msofficeforums.com/excel/2980-show-full-file-path-title-bar.html

If you (for some reason) work with two same-named documents on two different LAN shares, this will modify the Quick Access Toolbar to show the path:

1) Click the Office Button, then Excel Options.
2) Choose Customize in the left pane.
3) "Choose commands from:" All Commands.
4) Scroll down and highlight "Document Location" and Add and OK.