Not by visual difference - but perhaps by the ghost's reaction
If the ghost looks visually completely alive to Person B on the Material Plane, there isn't anything added by Person B now looking from the Ethereal Plane.
From the Etherealness spell (there are other ways to get there, but the rules are broadly the same):
You can see and hear the plane you originated from, but everything there looks gray, and you can't see anything more than 60 feet away.
So in fact there's even less visual evidence now that Person B is on the Ethereal Plane.
However, one thing might distinguish the ghost from the person. From the ghost's statblock:
Ethereal Sight. The ghost can see 60 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.
The ghost will be able to see Person B in the Ethereal Plane, so they might react to Person B's presence - while Person A can't see Person B and so won't react.
If the ghost is trying very hard to pretend to be a person, they may try to hide their reaction to Person B's presence. (If I were running it I would make them do a Performance check against Person B's passive Perception)
Poking them is usually easier
Planar travel is a fairly complex/expensive way to figure out if someone is a ghost, and needs you to be within 60 feet of them anyway. I would suggest that usually it's easier to just poke them to see if they are incorporeal.