This would be easy with FME Server, just set up a Directory Watch. On desktop it's a bit trickier. You could run a workspace on a schedule, say once per hour, that checks the date/time on the Excel file and if it's been changed in the last hour do the the processing. This wouldn't be instantaneous though, there's always an interval.

Alternatively, you could try a 3rd party app that does the checking for you and then run FME on the command line. There's a few discussed here. I have no experience with those though.