We show how shift theorem yields a routine quantitative test to determine whether a structure seen by two or more spacecraft is a quasistatic convecting object, such as a boundary layer. The test indicates the frequency range over which the data is consistent with a structure which is coherent between spacecraft, planar, and time independent in its rest frame. A cluster of four non coplanar spacecraft is required to determine the velocity of the structure in the spacecraft frame. Whether or not the data is found to be consistent with quasistatic convecting object, this analysis of the data from four non coplanar spacecraft yields the convection velocity of the plasma given the dispersion relation or vice versa, as well as the wavevectors corresponding to given frequency components. The test for coherence, planarity, and time independence is shown to be robust against detector and systematic inter-spacecraft timing and positional uncertainties. Random errors will affect a finite frequency range, in principle this can be determined to restrict the frequency range over which the test can be applied. ¿ American Geophysical Union 1993