we reply mainly by الحمد لله
we many times use these phrases:
الله يسلمك = Allah ysalmak: meaning may Allah protect you
تمام = Tamam: meaning good, fine
You can say:
الحمد لله، الله يسلمك
الحمد لله، تمام
And depending on the situation, the way we reply differs sometimes
For instance, we use greeting phrases to reply to such question in some situations like speaking on the phone (Like saying: أهلاً ,أهلا وسهلا, Ahlane أهلين)