acid test

noun3

1. •
   A test involving the reaction of an acid with another chemical.
2. •
   (figurative) A rigorous test or appraisal of the quality or worth of something.

   • [H]e reflected that few professional beauties could have stood, as this woman did, the acid test of that mercilessly brilliant morning. — 1912
3. •
   (historical) In the 1960s, a party based around the use and advocacy of the psychedelic drug LSD (known in slang as acid).