limus test

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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.

• Ending the fighting must be the absolute priority, the litmus test of the agreements' validity.     
• The success of wind power represents a litmus test for renewable energy.

... Cobuild

2. A test that uses a single indicator to prompt a decision: 

• "The word 'hopefully' has become the litmus test to determine whether one is a language snob or a language slob" William Safire.
  

2. (Fig) a question or experiment that seeks to determine the state of one important factor. 

• His performance on the long exam served as a litmus test to determine whether he would go to college. 
• The amount of white cells in my blood became the litmus test for diagnosing my disease.
  

... http://www.thefreedictionary.com/litmus+test