Is there any difference between flashbacks and imagination, and if there is, how do you know?
yes, flashbacks are based on memories while pure imagination is not
True. How we can make difference, then, when Asuka has memories, when she is imagining?
People say we can make difference between past and present time looking at hair of Asuka (longer and shorter).
If Asuka is imagining her mom while she is with senpai, what author made to show this difference, that it's not flashback...? Does anyone know, and can explain?
Well, it’s just an image of her mother—it’s not a memory of any specific event or incident.
Hypothetically I could say to every reader, “Think of your mother,” and everyone would get an image of their mother; it might be “generic the way your mother usually looks,” or it might be some more specific memory of her dressed a certain way or on a certain occasion. But we wouldn’t necessarily call those “flashbacks.”
But in a story, “flashback” is just shorthand for “not the narrative present.” A lot of the “flashbacks” in the story (the very brief ones) are sort of generic—Ayako thinks of Asuka being sick as a child, and it’s an image that stands for “the times Asuka was sick,” not for any one specific memory.
On the other hand, the first time Asuka thinks of Miyuki, it’s in reference to her having a hickey and lying about it.
Comics/manga have a bit of a problem visually representing habitual or continuing action compared to prose. You can just write, “Every Sunday we would have roast chicken, beef stew, or scrambled eggs.” But a panel either shows one of those things, or does something jazzy like showing a triple scene or something. The point is that visuals by themselves are immediate, they’re always “now,” unless there’s a verbal indicator or format cue (like a black border indicating a flashback).
That’s why overall context is so important. Remember, in “real time,” we’re still sitting with Asuka and Miyuki on a bench outside the wedding hall, waiting for Ayako to get done with her friends.