Open the hood and take a look. If the engine has a fairly narrow, black valve cover and the spark plugs are out in the open along the rights hand side of the engine, it's an M103. If the top is wide and silver colored and you can't see the spark plugs, it's an M104.
Question: Were you happy with the way it drove before you discovered what kind of engine you have? Be happy after.
The M103 is a very good engine.