I can tell 的含意

I can tell...
或
I can tell.
都不是slang.

I can tell he is sick.
照字面解 - 我看得出他是病了。[可能不是你所說的字面tell]

給你另一個例句：
I can tell you the truth, but I have to kill you.
我可以把真相告訴你，但之後我便要把你殺掉。[是一些電影對白]

I can tell！
作為單句或嘆句，意思是「我知道」或「我是知道的」

希望是你看到的這類句子所產生的疑問。

我估到啦/ 我明嘅....

I can tell from his face that he is sad.
or

I can see (from his face) that he is sad.

His face tells me that he is sad or unhappy.

2010-03-11 12:00:07 補充：
I can tell is not a slang