How does one differentiate a minhag, something that is not from a derabanan obligation? For exmple, how do we know the beating of the aravot is a minhag, but not a derabbbanan obligation? Weren't they both formed by rabbis of old?
There is an excellent discussion of this found at the Jewish Virtual Library. At the following link you will find each of the various terms used to discuss various aspects of the Oral transmission of Torah Law in the heading Dogmatics of the Halakhah.
In the paragraph just preceding the subject of Minhag and the section on Minhag itself you will find detailed explanation of the various uses of these two terms and how they differ traditionally.