I('ll) bet

1. I imagine or am fairly certain (that). The government is talking about another tax cut, but I'll bet us working-class stiffs don't see a dime of it. A: "Have you seen my glasses anywhere?" B: "I bet they're back in the restaurant—that's the last place I saw you wearing them." I bet he doesn't remember me—we only met briefly at a party a couple years ago.

2. That sounds correct; that's unsurprising. A: "Ugh, I'm not feeling very well." B: "Yeah, I'll bet, considering how much you had to drink last night." A: "I'm feeling pretty guilty about the whole thing." B: "I bet you are. It was a pretty low thing to do." Yeah, I bet you regret sneaking out now that you're grounded!

I/I’ll ˈbet (that...)

(informal)
1 I am certain (that...): I bet he’ll be late for the meeting — he always is.
2 used to show that you agree with somebody or are not surprised to hear something: ‘I’m furious about what she said to me.’ ‘I bet (you are)!’