"Telling" can have two meanings. First, it can mean "important," or "crucial," or "decisive":
"显著" or "重大"。
“When the lawyer asked if the witness needed glasses, it was a telling question: it completely changed the way the jurors thought about the case.”
律师问证人是否需要带眼镜时， 陪审员突然改变主意。 那个问题真重要， 真决定性。
There is also a second meaning。A question is "telling" or "revealing" if it reveals something significant (and usually bad) about the person who is asking it.
A: "How should we replace all the money that we will lose after we let women join our company?".
B: "That's a very telling question. It shows that you think that women will be bad for our company. You need to stop being sexist."