One of the conventions found in Beowulf that mark it as an epic is the presence of a hero of great stature. Stature in this case refers to both physicality and ability. Beowulf is a hero in battle as well as a heroic ruler.
An epic also involves supernatural forces. These are not hard to find as Beowulf’s main foe is Grendel. Grendel is a monster that eats warriors and was born of a dragon mother.
Another convention of an epic work is conveying to the reader a universal sense of struggle. An example of this is Hrothgar who speaks of the importance of maintaining prepared and capable warriors while at the same time also trying to maintain diplomacy with neighboring tribes. The universal human struggle here is how to negotiate the line between preparedness and instigation.