Acid test

The phrase "acid test" refers to a method of determining the true nature, value, or quality of something. It originates from the practice of using nitric acid to test the authenticity of gold, as acid would not react with genuine gold but would dissolve other metals or alloys. Figuratively, an acid test refers to a rigorous evaluation or examination that reveals the essential qualities or characteristics of a person, thing, or idea, often under challenging or difficult circumstances. It is a decisive or pivotal test that separates genuine worth or ability from superficial or false aspects.