The Mishna (Gittin 26) says that R’ Eliezer does not allow gittin to be written in advance “she’ne’emar ‘v’kasav lah’ – lishma.”The gemara asks: the din of lishma applies m'doraysa only to writing the toref of the get, i.e. the names, the date, the place -- not the tofeis, the boilerplate language. R’ Eliezer did not allow anything to be written in advance, including the tofeis. How does the derasha explain why that’s not allowed?
The gemara answers by amending the text of the Mishna to read “m’shum she’ne’emar” instead of just “she’ne’emar.”Explains Rashi: “she’ne’emar” implies that we are dealing with a din d’orasya – the derasha is the source for the halacha. “M’shum she’ne’emar” means that the derasha is just the motivation behind a din derabbaban. Since there is a derasha d’orasya that does not allow the toref to be written in advance, the Rabbanan extended that din and disallowed even the tofeis being written in advance.
If we can generalize this as a rule, then whenever the gemara or a Rishon quotes a din based on what sounds like a derasha, whether the source uses the term “she’ne’emar…” or “m’shum she’ne’emar” will clue you in as to whether it is a real din d’orasya or just an asmachta / din derabbanan. The question, of course, is whether the generalization holds up in all places, and it helps to be a baki like R' Ovadya was to figure that out!