At the ending of Meet Joe Black - how did Susan understand that Joe Black was Death, and that the guy she met at the coffee shop had actually died, and has now returned, by being swapped with Death?
The film's script gives us some insight into what's going on. In short, the 'Young Man' has different mannerisms and expressions than the Joe that she's come to know. She recognises those.
He smiles quizzically, hasn't quite heard her, stands right in front of her, loose, smiling, disoriented and yet so appealing. They are riveted on each other, uneasy and yet close.
SUSAN (cont'd): You're here...
He is trying to get his bearings.
JOE -- You bet.
Something about him makes Susan slightly tentative.
SUSAN: Where did you go?
Joe shrugs, scratches his head endearingly, uncertain of time and place.
JOE: I don't know -- y'know, I don't know -- it's all blurred up and hazy. And would y'know what I mean if I said I don't think it's worth figuring out?
Some realization is dawning on Susan, it renders her lightheaded.
JOE (cont'd): ...But now I'm back.
Susan regards Joe intently, searching his face for an answer.
SUSAN (gently): That's it?
JOE: Well, I don't know what else to say. It's a helluva party --
SUSAN: You think so?
JOE: Yeah...and you're the prettier thing here.
Susan blinks, a long silence, she touches the sleeve of his jacket, now her hand traces the outline of his face, she regards him intently and the dilemma she has been struggling with the last moments fades away.
Susan slowly realizes this is the Young Man. She is shaken, a sudden intake of breath.
It's not really clear here what Susan knows (or thinks she knows), but she evidently understands that something weird has happened, like waking up from a dream. In the end they pledge to work it out between them.
SUSAN: ...We know so little about each other.
YOUNG MAN: But we've got time.