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
Lane	LAYN	English
Viviane	-	French
Nuno	-	Portuguese, Medieval Portuguese
Sari (1)	SAH-ree	Finnish
Leonardo	le-o-NAHR-do (Italian, Spanish), lee-ə-NAHR-do (English)	Italian, Spanish, Portuguese, History
Petronia	-	Ancient Roman