acid test 
noun [in singular]
A conclusive test of the success or value of something: gritstone is the acid test of a climber’s ability...
- When you apply the acid test of value for money, hardly any make the grade.
- It can be used as an acid test of whether a person has interest in knowing or may be blinded by ideological convictions.
- And that, according to any number of people here, is the acid test of the world's new-found determination to stamp out doping.
Origin
Figuratively, from the original use denoting a test for gold using nitric acid.