Wiktionary
fair bet
n. An assumption that is likely to be true.
Usage examples of "fair bet".
It was a fair bet one of them was using Artifice on me but, oddly, I didn't feel particularly threatened.
It was a fair bet that the others, like Lando, made their primary living by winning at sabacc.
It is a fair bet, too, that if Ratnadatta collected her an hour or two earlier that is where he would have taken her.
Since Willi was the Company accountant, it was a pretty fair bet that the bid was in.
In fact, it was a pretty fair bet to say that you could have heard an individual atom of oxygen hit the floor.