Collins COBUILD Advanced Learner's English Dictionary - litmus test

(litmus tests) If you say that something is a litmus test of something, you mean that it is an effective and definite way of proving it or measuring it. Ending the fighting must be the absolute priority, the litmus test of the agreements’ validity... The success of wind power represents a litmus test for renewable energy. N-COUNT: usu sing, usu N of/for n