The phrase “How are you?” in America is often used when people say hello. A typical greeting is “Hello, how are you?”
When greeted in this manner, the most expected response is “Fine, thank you, and how are you?” To which, you will receive the response “Fine, thanks.”
It is actually a very odd conversation, because neither the first person nor the second person really is expecting an answer to the “How are you?” portion of the statement. In fact, if someone actually responded truthfully, and said “I’m feeling a little ill today.”, or something of that sort, it would likely make for an awkward start to the conversation.
The addition of “How are you?” to the greeting is simply an attempt at being polite, and not meant for a fully truthful answer. Following the obligatory “Fine, thanks.” portion of the greeting, the conversation can truly begin.
As with most everything in America, there are exceptions. If the person you are talking with is a friend, and you have had recent conversations about some aspect of your life that was going well or poorly, that person may expect a truthful answer to “How are you?” If this is the case, the word “are” may be emphasized a bit. “How ARE you?” This would show genuine interest in your well being. The person asking would not mind hearing that you are enjoying your new job, or that your back pain is getting better.
Americans are generally friendly people, and most like to be polite. If you are unsure of the intent of their “How are you?” simply respond with a “Fine, thank you.” If the person persists with asking further about your well being, then you can be comfortable that they are sincere about the question and can elaborate if you wish.