FOLLOWING the first report1 of an object of planetary mass orbiting a pulsar, Wolszczan and Frail2 have now reported the even more surprising discovery of two planet-size companions in orbit around the nearby millisecond pulsar PSR1257+12. The orbital periods of the two planets are about 98 days and 67 days, very close to a 3:2 ratio. Here we point out that, because of this near commensurability, the mutual gravitational perturbations of the two planets should produce not only small secular changes, but also larger periodic changes in their orbital elements. In particular, we find that changes in the eccentricities and orbital periods should become measurable within a few years. Such a measurement would help determine the three masses in the system and the inclinations of the orbits. More importantly, a detection of these changes, if they accord with the theoretical predictions presented here, would provide irrefutable confirmation that the periodic residuals observed by Wolszczan and Frail are indeed caused by orbiting planets, rather than some other effect. For the single planet-size object previously reported1 around the pulsar PS R1829-10, there is no dynamical test analogous to the one proposed here to confirm the planetary interpretation.