Usually for me if its and "epic" its pretty long and/or just sounds fucking epic. Its usually the long'uns though.
Bathory- A Fine Day to Die
Nile- Unas Slayer of the Gods, To Dream of Ur
Metallica-...And Justice for All, Orion, ALLL WITHIN MAH HAY-YAAAANDZ
Opeth- Black Rose Immortal, When, Deliverance
Summoning- Land of the Dead, or any song of theirs tbh
Jesu- We All Faulter
Khanate- All their songs are pretty long and epic-y IMO
Iron Maiden- Rime, Seventh Son, Brave New World, Alexander the Great
Burzum- Det Som En Gang Var, and the ambient one with the stupidly long name
Katatonia- Brave
Kvist- Min Lekam Er Meg Blott En Byrde
Setherial- For Dem Mitt Blod
Skepticism- Sign of a Storm, Pouring, The March and the Stream, Nothing
Thergothon- Elemental
Abigor- Emptiness / Menschenfeind / Untamed Devastation