You can say 'who is she kidding?' or 'who is he trying to kid?' if you think it is obvious that someone is not being sincere and does not mean what they say.
[informal] She played the role of a meek, innocent, shy girl. I don't know who she was tryingto kid.
'Maybe tomorrow will be better,' I said, hanging up. 'Who am I kidding?' I told Wolfe.'Tomorrow will be more of the same.'