Who and whom usage in single-word-sentence

I know that if you say something with only one person—for example "Look over there, it's Jack"—the correct response is "who?" (short for "who is Jack?"), or at least I'm pretty sure this is true.

Anyway, what I am asking is whether you can differentiate between the object/subject of a statement using either who or whom. In other words, if someone said, for example, "George just hit Jack!", could you respond with either "Who?" or "Whom?" to specify whom you are referring to?

So if someone said, "George hit Jack", could you say "Who?" as in "Who (hit Jack)?" or "Whom?" as in "(George hit) Whom?" to specify whom you are referring to?

Thanks.