1） Are you kidding=Are you pulling my leg??你在开玩笑吗?

example:

-do you know that Andy is going out with someone?
-what? are you kidding? i am still a bachelor.
-so who is that girl walking along the street with you yesterday?
-my god! she is my sister and she has got married and her baby is 5 years old!
-wow, your sister looks so young.

2） No kidding!

我不是在开玩笑.=I am serious!

example:

-Andy, you didn't pass the exam!
-I know you are kiddding.
-No kidding.