You never know. 也是“很难说”的意思
示例
A：Do you think he is willing or forced to do it?
B：Hard to say!
A：你认为他是自愿的还是被迫的？
B：很难讲！
A：Will she come?
B：Well, it's hard to say.
A：她会来吗？
B：呃，这个很难说。