You had better believe it!
you('d) better believe it
Absolutely, without question; rest assured. A: "Man, he's the fastest runner this school has ever had!" B: "Yep, you'd better believe it!" A: "And is the movie really scary?" B: "You better believe it!"
You (had) better believe it!
exclam. It is true without question! Yes, this is the best, and you had better believe it!