Alright, so when asking if someone wants something do you always use ほしがっています? Specifically, I'm thinking when you're trying to communicate with someone that cannot communicate back what they want. (Example: Does my dog want a bone?) or (Does baby want a bottle?) Could you say 私のいぬがほねをほしがっていますか。？？Or 赤ちゃんはほんゆうびんをほしがっていますか。I know expressing desires is kind of tricky.
ほしがっていますか？ works when you ask if it's simply the case or not (edit: in this case, it means "is he asking for?", not directly "does he want?") except some cases where essential difference between "want" and "ほしがる" does harm.
On the other hand, when you have judged that someone should want something from the situation and try to confirm it, you say ほしいのですか？ with のだ form.
Edit: I'm going to explain about "difference between "want" and "ほしがる".
Since ほしがる basically means "to show one's desire", it produces these differences.
- 水がほしい人いますか？ : Anybody wants some water?
- 水を欲しがってる人いますか？ : Did you find anyone want some water? (or "Among you, is there anyone itching to have some water? (Then, I'll punish you)")
So, ～がる doesn't really work as it would be from learner's viewpoint in these examples.
- （その人は）欲しがったから買ったのだ: He bought it because he asked for / because someone else wanted
- この子は将来医者になりたがってます: He is itching to become a doctor in future