Not if they were actually copied, or copied and modified. Copying formulas,
values or even code doesn't leave tracks.

Your dilemma is just one reason we don't knowingly provide assistance with
what are obviously homework problems.

You might consider giving a little 'pop quiz' every once in a while that
focuses on one or two areas that the main test did, just to check your
student's "retention" factor <g.
"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?