In some fanfictions, Naruto's pain might manifest through his actions. For example, he could become withdrawn and isolate himself from his friends. This is because Naruto has always been a social person, so any deviation from his normal behavior could be a sign of his internal pain.
One possible cause could be his sense of failure. Naruto always wants to protect his friends and village, so if he fails in a mission or is unable to save someone, it can cause him great pain.
In many fanfictions, Naruto might hide his pain by putting on a big smile. He's always been the type to try and be positive in the face of his friends. He could also distract others with his goofy antics, like making a joke or doing something silly to shift the focus away from his inner turmoil.
His fighting style could become more aggressive. Without the fear of pain, he might be willing to use more extreme techniques that would otherwise be too dangerous because of the pain they cause to the user. He could push his body to the limit more often.
He might show his scars to Sasuke. Given their complex relationship, it could be a powerful moment between them, a way to show Sasuke that Naruto has also faced great pain in their separation and the events that followed.
Naruto could be afraid of being seen as weak. In the ninja world, weakness is often frowned upon. He has a reputation to uphold, especially in front of his family. He doesn't want them to think that he can't handle his problems, so he hides his pain. He also might be trying to be strong for his family. If they see him in pain, it could affect their morale. So, he pretends that everything is okay to keep their spirits high and to give them the impression that they can rely on him no matter what.
In the context of fanfiction, it might also be a plot device to explore Naruto's vulnerability. It could lead to new character developments, such as others seeing him in a new light. Maybe it causes some characters to re - evaluate their relationship with him, while for others it could be a source of inspiration, seeing that even someone as strong as Naruto has his own set of hardships.
One common way is through loyalty. He'll always stand by his brother no matter what. Another is by listening. He gives his brother his full attention when he has something to say.
First of all, his friends would likely be very curious. Since Naruto has had a rather eventful childhood, the fanfiction could contain a lot of personal elements. They might be amused by his childish handwriting or the simple way he used to express his ideas. Some might even be touched if the fanfiction reveals Naruto's hidden insecurities or hopes from back then. It could also lead to his friends having a new perspective on Naruto, seeing a side of him that they didn't know before.