同意曾老师解答：

tell  vt.判断;断定;看出(英文解释： If you can tell what is happening or what is true, you are able to judge correctly what is happening or what is true.)

It was already impossible to tell where the bullet had entered.

已经无法判断子弹是从哪里打进来的。

I couldn't tell if he had been in a fight or had just fallen down.

我不知道他是和人家打架了，还是仅仅摔了一跤。

No one could tell exactly what was going to happen.
谁也不能猜出到底会发生什么情况。

No one can tell where or when or how he will die.
没有谁能知道自己在何处、何时及怎样死亡。

Can you tell which is which?
你能看得出哪是哪吗?

It's impossible to tell who will win the next election.
下届选举谁能获胜无法预知。

I can tell (from the way you speak) that you are from Australia.
根据你说话的方式,我能判断你是澳大利亚人。

从各位老师的解答可以看出，即使同一个动词，不同语义也会导致不同的用法，不能泛泛而谈地去讨论一个动词的语法。广大网友应该注意这一点。

tell 后一般不直接跟 that 等引导的从句，最好把 tell 改作says ，或在 tell 后面加上间接宾语(如 her, me,或 his friend)。例如：Tell him from me that he must be at the meeting on time.请转告他，叫他务必按时到会。

一般不说：

He often tells that he is going to achieve success.

可以说：

He often tells me that he is going to achieve success.

He often says that he is going to achieve success.

这两句都意为：“他经常告诉我说他将取得成就。”

但在下面句子里的 tell that ... 很自然，这里的意思是“了解”、“明白”，常与cannot 等连用。例如：His manner is quite normal again now, and no one could tell that only a few minutes ago he was shaken by such a great emotion. 他的态度现在又完全恢复正常了，没有人知道仅在几分钟前，他曾情绪激动得发抖。

    （以上均摘自《薄冰英语惯用法词典》）