Before plunging into the technical details, a few introductory words are in order.  Greek and Latin poems follow certain rhythmic schemes, or meters, which are sometimes highly defined and very strict, sometimes less so.

From the time of Homer—the composer of the Iliad, the oldest surviving Greek poem—epic poetry was recited in a particular meter called the dactylic hexameter.  It is fair to say that the dactylic hexameter defines epic: it is impossible to conceive of an epic poem not composed in hexameters.  The hexameter rhythms, when heard, signal that the poem being recited is an epic of some sort.

It is true that in Homer's era, epics were more sung than recited, to the accompaniment of a lyre.  However, this was not the practice in Vergil's day, when the spoken word was preferred.