An “acid test” is a foolproof, conclusive test of success or value, or which proves a hypothesis or the validity of an idea.
The product looks great, but will people buy it? That’s the acid test.
He played that game well, but the acid test will come when he plays a stronger opponent.
The acid test for the new theory will come when it is actually tried.
The origin of the phrase is a chemical test to prove the pureness of gold. This test was necessary to allow prospectors and dealers to distinguish gold from base metal. It was developed in the late 1700s and used nitric acid, which dissolve other metals more readily than gold. So the amount of metal dissolved would prove the pureness of the gold.
The figurative use started around the mid-1800s, with the earliest print reference being in the Wisconsin paper The Columbia Reporter in November 1845. “Twenty-four years of service demonstrates his ability to stand the acid test, as Gibson’s Soap Polish has done for over thirty years.”
The phrase also came into use in the 1960s for something entirely different. Ken Kesey and his Merry Pranksters held “Acid Test” parties in San Francisco. Those attending were serenaded by The Grateful Dead and given drinks of Kool-Aid (originally called “Fruit Smack”) spiked with LSD, known as “acid.” We don’t know whether it was the quality of the LSD being tested or the tolerance of the guests.
Or, perhaps, the neighbors.