litmus test

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

A test for chemical acidity or basicity using litmus paper.

The noun has 2 meanings:

Meaning #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
  Synonym: litmus

Meaning #2: a test that relies on a single indicator