Speckle interferometry (SI) is used for the measurement of the shape change of x-ray mirrors. Initially flat under thermal equilibrium, the mirror, or "thermal bender", is deliberately and adaptively bent by means of well-controlled temperature gradients. As the deflection of an optically polished surface can be obtained by a number of methods, the choice of SI and its subsequent advantages are discussed. Quantitative results are reported, referring to four kinds of tests: conformity, stability, sensitivity and repeatability tests. SI is recognized to meet the expectations: it provides a simple, complete, sensitive and accurate control of the shape of the bent x-ray mirrors.