The functional correctness of a software application is, of course, a prime concern, but other issues such as its execution time, precision , or energy consumption might also be important in some contexts. Systematically testing these quantitative properties is still extremely difficult, in particular, because there exists no method to tell the developer whether such a test set is "good enough" or even whether a test set is better than another one. This paper proposes a new method, called Multimorphic testing, to assess the relative effectiveness of a test suite for revealing performance variations of a software system. By analogy with mutation testing, our core idea is to vary software parameters, and to check whether it makes any difference on the outcome of the tests: i.e. are some tests able to " kill " bad morphs (configurations)? Our method can be used to evaluate the quality of a test suite with respect to a quantitative property of interest, such as execution time or computation accuracy.