A corrupted Naruto fanfiction might show Naruto or other characters using dark magic or evil powers in a way that goes against the original story's concept of chakra. If the story is full of plot holes and the characters' actions seem to have no logical basis, it could be a sign of a corrupted work. For instance, if Sasuke suddenly teams up with the villains for no good reason. Additionally, if the fanfiction has a lot of OOC (out of character) dialogues where the characters speak in a way that doesn't match their personalities at all, it's likely to be corrupted.
One sign could be that Gaia always tries to keep Naruto close to her. She might constantly be around him, showing up wherever he is. Another sign could be that she gets jealous when other characters interact too much with Naruto. For example, if another female character shows interest in Naruto, Gaia might act in a way to drive that character away.
Might Guy is also a top choice. He is Lee's teacher and has mastered the same style of taijutsu. His experience and his ability to open the Eight Gates make him an extremely powerful fighter in hand to hand combat. In fanfiction, he could be shown in epic battles, using his full power against strong adversaries.
Master Hand could be an ancient being that has been observing the Naruto universe from afar. Maybe he's a sort of guardian of the balance between different worlds, and when Naruto's situation threatens to disrupt that balance, he steps in.
One interesting scenario could be a fight between Naruto and Sasuke where they rely solely on hand to hand combat skills, without using any ninjutsu or genjutsu. It would be a raw display of their physical strength, speed and martial arts training.
One sign could be that Tsunade is overly protective of Naruto in the fanfiction. She might always be looking out for him more than she does for others. Another sign could be that she gets flustered or blushes around him, which is a common indication of having a crush.
One sign could be that Kyuubi gets jealous when other characters get too close to Naruto. Maybe it tries to interfere or make its presence known in a way that scares off the other characters. Another sign is that it might be overly protective of Naruto in dangerous situations, more so than just a normal sense of helping. It could also be possessive in the way it shares its power with Naruto, as if it's something only Naruto is entitled to among all the characters in the fanfic.
If in fanfiction Naruto shows a lack of empathy towards others, that could be a sign. For example, if he ignores the pain of his friends or villagers without any remorse. Another sign could be if he manipulates people for his own gain without any regard for their feelings.
One of the signs could be the way Anko looks at Naruto in fanfic. There might be a certain softness or longing in her eyes. Also, she could be very eager to engage in conversations with him, always asking him about his day or his latest mission. She might remember small details about him that others wouldn't. And when Naruto is in danger, Anko could show extreme concern, even more than what is considered normal for her character in the original series. These are all possible signs of her having a crush on Naruto in fanfic.