litmus test

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A test for chemical acidity or basicity using litmus paper.
2. n. 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).

Wiktionary

1. n. chemistry A simple test for the acidity or alkalinity of a substance, using litmus paper.
2. n. figuratively, by extension Any test which produces a decisive result by measuring a single indicator.

GNU Webster's 1913

1. (Chem.) A test to determine acidity or alkalinity, using litmus as an inddicator.
2. (Politics) A test for a single factor, which has only two outcomes, positive or negative 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.

WordNet 3.0

1. n. 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
2. n. a test that relies on a single indicator