I am told, (as I am quite often) that this isn't especially coloquial, and may be a bit too formal, but okagesama de, genki desu. Is the standard reply.
If you are seeing someone (again) who've you've seen recently, a more natural way to ask how they've been is genki deshita ka?... Unless you prefer the simple genki desu ka?... Since you're prefixing words w/ O- the formality of my response above shouldn't be an issue for you!