When to use usted

[jrhjazznblues]

Every translation that I have seen says that the formal way to say "How are you?" in Spanish is "Como está usted?". Is it necessary to say usted if it is clear who I am talking to? Can I just say "Cómo está"?

[Rusty]

You can always omit the subject pronoun if your interlocutor knows whom you're addressing.
The subject pronoun is only needed in the third person, and only when context doesn't rule out any other third-person possibility.

Make sure you use the opening interrogative mark when writing a question, and add an accent to the interrogative. ¿Cómo está?

[jrhjazznblues]

You're the man.

[AngelicaDeAlquezar]

I agree with Rusty that, normally, pronouns are used for emphasis or to avoid confusion as to whom or about whom one is speaking.
Yet, sometimes it's more polite and formal to say "¿Cómo está usted?" than "¿Cómo está?"; that is why most textbooks use this formula.

- Buenas tardes, Don Pedro, ¿cómo está usted? / ¿cómo está? -> Both would be equally polite, as you've already used the respectful title "Don" and the name of the person.

- ¿Cómo está, señora Martínez? -> Polite way to ask, addressing the person by their name.

- Buenos días, ¿cómo está? -> Depending on the tone of the greeting, this could be polite enough, but if you want to show some more respect, or don't know the name of the person, as in the previous examples, "¿cómo está usted?" might sound better for the person you're greeting. :)