Use 'no kidding' to reinforce something that is clearly true or apparent to everyone. It's hot outside in July. No kidding, we're in Texas.
Pro Tip 2/3
Sarcasm Indicator
'No kidding' can indicate sarcasm when responding to obvious statements or questions. We lost the game 10-0. No kidding, it was a disaster.
Pro Tip 3/3
Informal Agreement
'No kidding' is an informal way to express vehement agreement with what has just been mentioned. This book is really boring. No kidding, I couldn't even finish it.