use of litmus paper to test the alkalinity or acidity of a solution (Chemistry); test in which one particular event or quality serves as the decisive factor (Figurative)
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