a litmus test

something that shows clearly what someone's opinions or intentions are

Usage notes: Litmus is a substance used in chemical tests because it changes colour.

His views on abortion are effectively a litmus test of his views on women's rights.

Name	Pronounced	Usage
Middleton	['midltən]
Ilse	IL-sə (German)	German, Dutch
Sebastian	ze-BAHS-tee-ahn (German), sə-BAS-chən (English), se-BAHS-tyahn (Polish), SE-bahs-tee-ahn (Finnish)	German, English, Swedish, Norwegian, Danish, Polish, Finnish, Romanian
Rens	-	Dutch
Bowers	['bauəz]
Marijeta	-	Croatian