have a joke

除了楼上们所说的，还有一句习语也可以：pull one's leg?
你开玩笑吧？are you pulling my leg?

play a joke on sb

你在开我玩笑吗？——Are you kidding me?
我在开你玩笑呢！——I'm kidding you.
你是在开玩笑吧?——Are you kidding?

to play a joke on sb
make fun of sb
他们拿那位老人开玩笑。
They are making fun of the old man.

他喜欢拿他的同学开玩笑。
He likes playing jokes on his classmates.

Are you joking?
你在开玩笑吗?