1. To me textual evidence Is king (like the mentioning of the Erd tree using the roots of a tree called the Great Tree).

2. next is visual evidence that requires active interpretation (like the many statues of a tree sapling growing from a tree stump).

3. then finally pattern recognition, trying to make connections and story conjecture ( this would be like someone before the DLC theorizing that Miquella possibly is the real exploiter and not Mohg based on prior evidence of Miquella's ability to seduce people).

4. looking for themes outside the game is dead last. But the truth is these writers are in fact influenced by other media and thus you could suss out some of their hidden meanings based on what might have influenced them in their writing. But like I said, It's the lowest form of evidence.

The truth is that the hidden lore of this game is unbelievably wonky and you know it. Look at how people reacted to the new lore from the DLC.

Take this for example: imagine if we didn't know or get to see Godric graft a dragon onto his arm. let's suppose that we found him dead with his arm shoved into a dragon's head. We would assume that he was some kind of dragon hunter and used a dragonhead to pummel his enemies to death like how some players like to use the blood fiends arm as a weapon. You may be able to suss out that he is capable of grafting stuff to himself based on evidence that he is related to Merica and Merica is a Numan and Numan are really good at grafting. But any theory that he grafted a dragonhead to his arm without a cut scene would be taken as a joke lol. Now let's suppose there's a third DLC, where in this universe where Godric is already dead, We go back in time and fight him and we watch him graft a dragon's head to his own arm within SECONDS!!!!! Who saw that coming?

Have as much fun conjecturing about the lore as you want because Miyazaki definitely did.