The term "acid test" has its origins in the gold mining industry. In order to determine if a particular rock contained any gold, it would be placed in an acid solution. If the rock dissolved in the acid, then it was considered worthless and discarded. However, if the rock did not dissolve, it was deemed valuable enough to warrant further testing.
The phrase "acid test" has since come to mean any rigorous test or standard by which something is judged. It can also refer to a situation in which only the most essential and important things are kept, while everything else is discarded. In other words, it's an ultimate measure of quality or effectiveness.