the acid test
◄◄◄ If you refer to something as the acid test, you mean that it will show or prove how effective or useful something is. You can also say that something is an acid test.
The acid test for the vaccine will be its performance in African countries where malaria is raging more fiercely than in Colombia.
So far, I don't feel too bad but I'm waiting for my first really stressful day when things go wrong. That will be the real acid test.
The case, as a whole, is an acid test of the justice system.