It depends. Sometimes it might be obvious if major elements are missing or if there's a sudden change in the narrative. But in some cases, it could be harder to tell.
I'm not sure if there is such a specific fanfiction readily available. Fanfiction can cover a vast range of ideas, but this one might be quite rare or even not exist in mainstream fanfiction repositories. You could try searching on some specialized fanfiction websites, but keep in mind that it should adhere to the rules and ethics of those platforms.
In such fanfictions, Percy's character is often explored through his resilience. For example, if he's physically abused, we see how he still tries to fight back or find a way to escape, which shows his determination. His loyalty also comes into play as he might not want to burden his friends with what he's going through.
There could be. Fanfiction writers are known for their creativity and ability to come up with the most unexpected storylines. However, given the relationship between Percy and Hera in the original context, it would be a very different and perhaps controversial take. It would be best to search through popular fanfiction platforms to see if such a piece exists.
It could be the traditional villains from the original story. Maybe some of the old enemies of Percy who have a vendetta against him. For example, the Titans or some rogue demigods who oppose him for power or personal reasons.
Well, one possibility is that Percy's friends try to find a cure. Maybe Annabeth uses her wisdom to research in the library of Camp Half - Blood while Grover tries to use his nature - related powers to find some special herbs. And Percy is just lying in his cabin, feeling really weak and feverish.
One common theme could be the exploration of Percy's inner strength. Through the torture, the story might show how he endures and overcomes. Another theme could be the reaction of his friends. They might be desperate to save him, highlighting the bonds of friendship. Also, it could explore the motives of the torturers, whether it's for revenge or some other dark purpose.