you can never tell 你永远无法分辨；谁都说不准；不可预料

【例句】
Some people are so terribly insincere you can never tell if they are telling the truth.
有些人非常虚伪，你根本说不准他们是否在讲实话。

When will he arrive?-You can never tell.
他什么时候到？——说不准。