Is there a way to tell if I'm in a combiner or a reducer?

[W.P. McNeill]

I have a MapReduce process that uses the same class for its combiner and
reducer.  I just realized that I want the behavior in the combiner and
reducer to be slightly different in this one place.  I could write separate
combiner and reducer classes derived from a common source, but in my
situation this is overkill.  Is there some way I can tell at runtime whether
I'm running as a combiner or a reducer?  Like some configuration setting?