How?! I could tell by the direction alone. Then the writing, you could clearly separate what was Ntozake's and what was his. And dear God, the scene where Janet was on the phone at dinner made me sick to my stomach. "Baby I know we had a fight last night..." Really, bitch? Who says that?
I agree about Janet's acting, but I blame Tyler. All of the actresses struggled to a certain degree, some more than others, because of the material they were given. You can't do but so well if your character has no sense of individuality and has lines that vague.
Dead horse, Janet needs to get away from Tyler. If she can find a movie that gives her a more unique character and doesn't focus on a million other characters at the same time, I think she'd surprise us all. Evidence is in her first crying scene, when she was going on about her specific pains in reference to Kimberly, I honestly believed it.