litmus test

lakmuskoe

koetinkivi puhekieltä A simple test for the acidity or alkalinity of a substance, using litmus paper.
puhekieltä Any test which produces a decisive result by measuring a single indicator.
1940, Allison Danzig, "Leading Elevens Face Stern Threats in Saturday's Games," New York Times, 2 Oct., p. 27,

Temple, under its new coach, Ray Morrison, formerly of Vanderbilt, will be put to the blue litmus test Friday night when it takes on Georgetown.

For good reasons or bad, early membership of the currency union has become the litmus test of being a "good European."

He opposes a constitutional amendment to ban same-sex marriage—a litmus test for many social conservatives.