If the students actually used entire workbooks (templates) from online, you
might take a look at the file's Properties and see who the author of the file
is. That might tell that someone other than the student originated that
particular file.
Excel 2003 & earlier:
File -- Properties and look on the [Summary] tab.

Excel 2007:
Office Button -- Prepare -- Document Properties


"Mr. G" wrote:

I am grading tests done on EXCEL. Several students' answers look almost
identical to homweork problems which had been posted online. Using such
templates is not allowed on my test. Is there a way to identify the sources
of portions of another EXCEL file that were copied from another file into the
students' test files?