He's upstairs doing his homework, believe it or not. 他在楼上做功课，信不信由你。
This is the third hotel we've tried - would you believe it?
After everything that had happened between them, would you believe it, she married him!
Believe it or not, people do help one another, even if it seems like everyone is cruel.
Those were his actual words, believe it or not.
See also: believe