"How are you?" or "How are you doing?" is basically either a greeting or a question, depending on who is asking and how you feel like perceiving/answering it. I answer, 87.3% of the time :-) "Pretty good" and move on. Sometimes, if I like/care :-) about the other person I'll add, "How about you?" or something of that nature. If I'm feeling down, I might answer, "Okay" in a more flat tone. If it is someone I think cares (spouse, therapist, very very good friends or relatives) I could answer, "Not too well, I just fell down the stairs and broke my neck the other day" (or whatever the actual event that has you not feeling well is :-)

The trick for me is to just throw something out there and move on or not, depending on how they take it from there. Someone who really wants to know the answer, who is asking the genuine question instead of just doing the standard greeting, will be more intense and stop and listen to your answer, etc. so you will know to give them a little more if you want.

It can be a bad idea to start by taking it as a question though; my husband's ex-wife sometimes does that and a mutual friend once said, "Don't ask her 'How are you?' or she'll tell you!" Especially if you are feeling negative or down, not realizing another is just acknowledging you, greeting you, and starting in on your current life's woes until their eyes glaze over can be embarrassing. Establish your presence first, with "Okay" or something else neutral like that (or even, "I could be better") and see if they linger or ask for details.