Another way is through his interactions with other characters. If there's a character who is also vying for Arthur's attention, Merlin's jealousy might be evident in how he tries to undermine or outdo that character. He might be more competitive or less friendly towards them, all because he's jealous of their relationship with Arthur.
It can be shown through his actions too. Maybe Merlin is a bit more snappy or short - tempered with Arthur when he's feeling jealous. Like when Arthur asks Merlin to do a task, Merlin might do it with a bit of an attitude instead of his usual eagerness, because he's jealous of something Arthur has or has done.
In such fanfictions, Merlin is often portrayed as a very powerful sorceress. She has an air of mystery around her. Her magic is usually depicted as extremely potent and she uses it to protect Arthur or further their shared goals. She might be shown as a manipulator at times, using her intelligence and magic to influence events in their favor.
Often in fanfiction, Ygraine is a complex character. She may be portrayed as a victim of Uther's desires in one aspect, but also as a strong - willed woman. When it comes to her relationship with Merlin and Arthur, she could be the one who unknowingly weaves their fates together. For instance, her death could be the starting point of a chain of events that bring Merlin and Arthur closer, and fanfiction writers can play with this concept in many different ways.
Often, the relationship is one of mentorship. A female Merlin in fiction might use her wits and magic to help Arthur make wise decisions. She may be more in tune with the subtleties of the court and the people, and thus be able to advise Arthur better. For instance, she could sense the unrest among the common folk before it becomes a big problem and help Arthur take preventive measures, all while having a more nurturing aspect to her mentorship compared to the traditional male Merlin.
In some fanfics, Arthur's heavier build might be shown through his broader shoulders and more muscular physique compared to Merlin's slighter frame. This can be used to symbolize Arthur's strength and power.
Often, it's shown through Peter's internal monologues. He might think about how Morgan gets to have a normal family life with Tony while he has to hide his identity and fight crime. For example, he could be seeing Morgan have a simple dinner with Tony and feel a pang of jealousy because he rarely gets such a normal moment with Tony.
Well, in these fanfictions, Sasuke's jealousy can be quite intense. It can be seen when he gets angry or distant when another male character is around Sakura. For example, if a male teammate gets too friendly with Sakura, Sasuke may start to give him the cold shoulder. It's a way to show that Sasuke has deep feelings for Sakura and doesn't like the idea of someone else getting close to her.
Garu's jealousy in Pucca fanfiction is often shown through his actions. He may become more withdrawn or brooding when he sees Pucca with another character. For instance, he might sulk in a corner or train more vigorously as a way to distract himself from his jealous feelings.
Merlin would likely be seen as a bit of an enigma at Hogwarts. His magic is different from the norm, so he might be a bit of an outsider at first. But his kind heart and loyalty would soon win over his classmates. Arthur would be the brave and headstrong character, perhaps a bit like a Gryffindor version of his usual self. He'd be eager to learn about combat magic and would probably be a natural leader in group projects or Quidditch teams.
Sakura's jealousy of Hinata can be portrayed in a more subtle way too. In some fanfictions, she might distance herself from Naruto when she sees Hinata getting closer to him. Instead of showing her jealousy outwardly, she'll withdraw and become more focused on her training or other friendships. But it's clear to the readers that she's jealous because of the way she'll steal glances at Naruto and Hinata when they're together, and the sad look in her eyes when she does so.
Often, his jealousy is manifested in his actions towards James. He might try to outdo James in academic or magical feats, all because he wants to prove that he is better than James, especially in Lily's eyes. And when it comes to Lily, he may be secretly longing for her attention while trying to hide his feelings because he knows she is with James.