I take it you're familiar with VBA modules:

in a code module, paste:
Sub test()
MsgBox "Path: " & ActiveWorkbook.Path & _
Chr(13) & "Name: " & ActiveWorkbook.Name & _
Chr(13) & "FullName: " & ActiveWorkbook.FullName
End Sub

then execute it.

It will show you what Excel sees as the name and path of the active workbook.
to make "double-sure" you COULD even test the ActiveWorkbook.FullName
to make sure that it is Exactly the file you're expecting them to use.

Like:

if (ActiveWorkBook.FullName <> "P:\user name\FileName.xls") then
msgbox "Copies of this file are not allowed"
exit sub
end if