That's my response here to the question: ¿Cómo estás? Usually followed by ¿Y tú? It's considered impolite not to answer here. Another common greeting is ¿Qué tal? (How is it going?) Same answer.

"How are you?" is better than one of the British alternatives: "Are you well?" This can cause some State-siders to wonder if they look "sickly".

Chuck