litmus test

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Rather, the principle is a litmus test.
当然了，原则就像一个石蕊试纸试验。
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It requires a "Litmus Test", which determines a "good service".
它还需要一个“石蕊试验”，以决定是否是“良好的服务”。
Often, she said, a kiss can provide a window into long-term compatibility. "A first kiss is nature's ultimate litmus test, " she said.
她经常说，亲吻能为长期的兼容性提供一扇窗“第一吻是自然界最终的石蕊测试”

I wish there were some easy litmus test that I could just give you so you'd know which of you would be making a mistake taking this class.
我希望能有一种简单的石蕊测试,让我可以直接展示给你们并让你们知道,你们中的谁上这门课是一个错误。
耶鲁公开课 - 死亡课程节选
In both of these cases, I take it, Socrates' point is that his own individual moral integrity stands as a kind of litmus test, you might say, for whether to engage or disengage from political life.
从这两场事件看来，苏格拉底的观点是,他自己个人的道德正直,经得起立见分晓的检验，我们可以看到无论参与,政治生活与否。
耶鲁公开课 - 政治哲学导论课程节选

But WikiLeaks will serve as a free speech litmus test for every company in its orbit.
FORBES: Apple Nixes WikiLeaks iPhone App. Will Google Follow?
Mr. MOTHERSBAUGH: Yeah, it was the one that was, like, the litmus test for us.
NPR: The '80s Strike Back: Devo's Mark Mothersbaugh

If you say that something is a litmus test of something, you mean that it is an effective and definite way of proving it or measuring it.