Common signs can include isolation. In the fanfiction, Sasuke may try to isolate Sakura from her friends and family. This makes her more dependent on him and easier to control. Physical aggression, even if it's just threatened rather than actually carried out, is also a sign. For example, if Sasuke in the story makes Sakura fear that he might harm her physically if she doesn't do as he says. These types of behaviors in the fanfiction would suggest an abusive relationship between Sasuke and Sakura.
Well, in some Sasuke Sakura fanfictions, a sign of an abusive relationship could be emotional manipulation. Sasuke might use Sakura's feelings for him to get her to do things she doesn't really want to do. For instance, he could guilt - trip her into staying with him when she has other opportunities. Additionally, if there is a lack of respect for Sakura's opinions or dreams in the story, like Sasuke always dismissing what she says without consideration, it can indicate an abusive relationship.
One common sign could be excessive control. For example, if Sasuke in the fanfiction constantly restricts Sakura's actions or who she can interact with. Another sign might be verbal aggression, like Sasuke yelling at Sakura in a demeaning way. Also, if there's a pattern of Sasuke ignoring Sakura's boundaries, such as physically getting too close when she doesn't want to, it can be a sign of an abusive relationship in the fanfiction.