You could do a search on files using a GREP type utility like the FREE one here, and search your suspect files for something like SHEET1 which should show in probably all Excel files. If not found it is probably some other type of file.

Agent Ransack - Search Utility - Freeware

Or, if you want to go all out and pay for it $2,495 you could get something like this:

Encase - is the industry leading computer forensic software tool

UltraEdit also works well as a KiXtart Editor and locating these file searches
UltraEdit - Text Editor - HEX Editor

I do like the looks of MvdW's code for finding the size though.

[ 11 September 2001: Message edited by: NTDOC ]