Whether or not you respond to such a question depends on who makes this comment and where this person does so.
For example, if you’re walking in an area you’re unfamiliar with or that isn’t safe, you shouldn’t reply. If you do, you may end up face to face with a robber or dangerous criminal.
On the other hand, if you’re on campus or at work and someone you don’t know asks “Hi, how you doing?,” you can politely smile or nod or say “Fine, thanks.” Either a quick non-verbal or verbal reply will suffice.
Although Americans are (at least superficially) friendly people, they often don’t expect a reply to this query.
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