你能看出他累了。

You ___could ___tell that he was tired.

你永远无法判断他是否认真。

You ___can never ___tell whether he's being serious or not.

我能听出那是一个男人的声音，但我没能识别出来。

I ___could ___tell it was the voice of a man, but I failed to identify it.