acid test

Từ điển Oxford Advanced Learner 8th

ˌacid ˈtest [acid test acid tests] BrE NAmE (also ˈlitmus test especially in NAmE) noun singular
a way of deciding whether sth is successful or true
•The acid test of a good driver is whether he or she remains calm in an emergency.
See also: ↑litmus test

Word Origin:
figuratively, from the original use which was a test for gold using nitric acid.

Example Bank:
•The perception of fairness is the acid test for democracy.
•This type of stone is the acid test of a climber's ability.