you never can tell/you can never tellTHINK SO/NOT BE SUREused to say that you cannot be certain about what will happen in the future 谁也说不准,很难说 The boy might turn out to be a genius. You never can tell. 这男孩说不定最后还是个天才呢,谁知道呢。
→ you never can tell/you can never tell at tell(SPOKEN SECT)