The Collaborative International Dictionary of English v.0.48:

litmus test \litmus test\,
   1. (Chem.) A test to determine acidity or alkalinity, using
      litmus as an inddicator.
      [PJC]

   2. Hence: (Fig.) A test for a single factor, which has only
      two outcomes, positive or negative; specifically:
      (Politics) the question of whether a candidate for office
      is for or against a particular position on an issue; --
      used, e.g. by voters concerned predominantly with a single
      issue, to decide whether to vote for or against a
      candidate; as, many voters take a candidate's position on
      abortion as their litmus test.
      [PJC]

WordNet (r) 3.0 (2006):

litmus test
    n 1: a coloring material (obtained from lichens) that turns red
         in acid solutions and blue in alkaline solutions; used as a
         very rough acid-base indicator [syn: litmus, litmus
         test]
    2: a test that relies on a single indicator; "her litmus test
       for good breeding is whether you split infinitives"