https://en.wikipedia.org/wiki/Acid_test https://en.wikipedia.org/wiki/Acid_test: > An acid test is a qualitative chemical or metallurgical assay utilizing acid

https://en.wikipedia.org/wiki/Acid_test https://en.wikipedia.org/wiki/Acid_test: > An acid test is a qualitative chemical or metallurgical assay utilizing acid. Historically, it often involved the use of a robust acid to distinguish gold from base metals. Figuratively, the term represents any definitive test for attributes, such as gauging a person's character or evaluating a product's performance. Specifically here, they're using the figurative sense of "definitive test".