there's no knowing/telling/saying

If you say there's no knowing, you mean it is impossible to be certain about something.

无从知晓;难以预料

She's very unpredictable, so there's no knowing how she'll react to the news.

她这个人很难预料，所以不知道她会对这个消息作何反应。

There's no saying what they might be capable of achieving.

他们能取得什么样的成就是很难说的。