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
Einion	-	Welsh
Jelka	-	Slovene, Croatian, Serbian
Joasia	yaw-AH-shah	Polish
Cristi	-	Romanian
UĞUr	-	Turkish
Souma	so:-mah	Japanese