how was I to know? idiom informal
（表示所做的错事并非自己的过错）我怎么会知道?
used to say that something you did wrong was not your fault because you did not have enough information
I just wanted to give her a surprise; how was I to know you'd already bought tickets? 我只想给她个惊喜，我怎么会知道你已经买了票？