It was being real of you to tell me this dress made me look fat. Thank you. I had no idea you thought that and I never would have known if you went on thinking I would take it personal.
Another way of saying are you being serious?, or are you being really serious, used commonly in a question to ask someone if they're being serious about something or someone. Are you being for real? is commonly also used if they're in shock about something or an situation.
"Are you being for real or are you acting dumb?"
"Are you being for real or are you just acting stupid?"