litmus testnoun [ C ]
uk/ˈlɪt.məs ˌtest/us/ˈlɪt.məs ˌtest/a test to find out if a substance is acid or alkaline
[ usually singular ] someone's decision or opinion about something that suggests what they think about a wider range of related things:
The president's policy on abortion is regarded as a litmus test of his views on women's rights.