How are you doing?是完整的问法.现在时态可变为How have you been?意思是一样的.
How are you?是省略问法.我们中国人使用最多的问法.
还可以用What' up?What's new?等询问"你好吗"?
你可以这样回答：
1.i am doing fine
2.i am doing well
3.i am doing great
4.just so so
5.fine,thank you.And you?