There are quite a few idioms where
tell is not used exactly the same as in
to tell someone something.
The following are usually negative and usually followed by an indirect question or a
that clause. These mean that something is not obvious or easily seen by the subject of the sentence.
can't tell / not be able to tell / impossible to tell (if ..., whether ..., how many ..., which ..., where ..., when ..., that ...)
I can't tell what it says on that sign. I must need new glasses.
Judy was unable to tell where the dress had been torn and repaired.
No one could tell that the violinist made a mistake.
___________
Note the idiom
not be able to tell the difference.
Sherry said that she had put too much salt in the stew, but I'm sure that nobody else could tell the difference.
___________
The affirmative form
can tell means that something is obvious, easily seen.
I can tell that you've passed the exam. You look very happy.
You can tell when the dog is hungry. He lies by his dish and looks up hopefully every time anyone passes by.
I could tell that Jake was nervous when he gave his speech.
CJ