So what's the difference between asking:
だれか？
And:
だれが？
2
Your question ultimately boils down to the difference between two particles: か and が.
か is a question marker, and an indefinite marker. After an interrogative pronoun like だれ ("who"), いつ ("when"), or なに ("what"), か forms the indefinite -- だれか ("*somewho" ⇒ "someone"), いつか ("*somewhen" ⇒ "sometime"), なにか ("*somewhat" ⇒ "something").
が is mostly a subject marker (and sometimes, in archaic constructions, a kind of possessive linker between nouns). だれが would be "who" as the subject of a sentence.
1
In both cases you are missing a verb, so please keep in mind that any answer assumes that the verb is being understood from context.
That being said, my reading of だれか？ would be to fill in with a verb that would cause the first part to be interpreted as "someone?" such as in "is someone there?".
My reading of だれが？would be closer to "who?" such as in "who did it?".
You may also refer to the definition from jisho.org of だれか as someone/somebody.
