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
Karthik	-	Tamil, Indian, Telugu, Malayalam
Margrit	MAHR-grit	German
Helen	HEL-ən (English)	English, Swedish, Norwegian, Danish, Greek Mythology (Anglicized)
Sunny	['sʌni]
Salambek	-	Chechen
Terrie	TER-ee	English