I'll add another information. どなた【何方】（代） ① 不定称の人代名詞。「だれ」の意の尊敬語。 「お客様は―ですか」「あの方は―様でしょう」 （Macの辞書より） You were taught "どなた" as a direct question, but it's not true. We can use it in both a direct and a third person question. As Ryoko san and Masaya san already explained, they are correct. I think that the most polite phrase is "失礼ですが、（あなたは）どちら様でいらっしゃいますか？” どちらさま＞どなた＞だれ

どなた is also used to talk about a third person. この方はどなたですか？ source(s) http://dictionary.goo.ne.jp/leaf/thsrs/5487/m0u/ I agree with Ryoko-san about the difference in politeness :-) どなたis politer thanだれ I think, generally speaking, when we start a sentence with a polite word, we ends with a polite word, so your idea is basically right. But I googled a little and あの方は誰and あの人はどなたboth seemed to be used by Japanese people. あの人はどなた was less common. "あの方は誰"　about 1.810.000 results "あの人はどなた" about 655.000results Examples, which are not what I made but what other Japanese people wrote online. 　 あの方は誰だろう？ あの方は誰でしょうか？ あの方は誰でしたっけ？ あの方は誰なのか。 あの方は誰ですか？ あの人はどなたですか？ あの人はどなただったのか… あの人はどなたなんでしょうか

I guess your question is really nice question. When i read the Japanese sentence, I can see the situation the sentence used was in the the situation that a lot of people are talking at the same time. When i refer a person who is away from me, i will say "あちらはだれですか？" in this sentence, the person has the idea " I do not mind about him but i ask it with my curiosity." because だれ is describing "who" and "what". the reason why i use あちら is because the thing which i indicated was a "person". we need to respect the person a little bit at least. When I refer a person who is closer to me and the words could be heard by him possibly, I would use "あちらのかたはどなたですか。” あちらのかた is really nice phrase for the person who heard the phrase. but recent days, this phrase is getting very polite and the person who heard this phrase will get angry a little bit because great respect can be a way to let the person who heard feel inferior because he did not have any idea to use. but When i usually communicate with people in fact, i will pay more respect to people if i take the potential fortune into consideration and said "あちらのお方はどちらさまですか。" I think this phrase is the most respectful phrase. On the other hand, the ordinary Japanese people dislike the person who use this phrase because it is extremely polite way to ask who that person is. (this feeling is similar to words used as if they were the person who speak British Royal English.) I hope this helps.

"あの方" and "どなた" both convey the speaker's respect to the unknown person (not to the person being spoken to). That is why you should use them in pair in that sentence to ensure consistency, as Ryoko-san mentioned. The speaker's respect toward the listener is expressed through "ですか". That is, you could say "あの方はどなただ？" if you have no intention to show your respect to the listener, but to show respect to the unknown person. Conversely, if you want to show respect to the listener but not to the unknown person, you could say "あいつはだれですか？" The Japanese keigo system is highly complicated, isn't it?

"あの方" and "どなた" both convey the speaker's respect to the unknown person (not to the person being spoken to). That is why you should use them in pair in that sentence to ensure consistency, as Ryoko-san mentioned. The speaker's respect toward the listener is expressed through "ですか". That is, you could say "あの方はどなただ？" if you have no intention to show your respect to the listener, but to show respect to the unknown person. Conversely, if you want to show respect to the listener but not to the unknown person, you could say "あいつはだれですか？" The Japanese keigo system is highly complicated, isn't it?