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!"
See also: believe, better
Farlex Dictionary of Idioms.

You (had) better believe it!

exclam. It is true without question! Yes, this is the best, and you had better believe it!
See also: believe, better
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • You better believe it!
  • you('d) better believe it
  • you/you'd better believe it!
  • you'd better believe it
  • bend over, here it comes again
  • BOHICA
  • I can't believe it/that/this
  • I can't believe!
  • hand over the baton
  • pass the baton