A version of the local state isomorphism theorem is used to show how nonlinear model structures, within some specified class, can be tested for indistinguishability from some given structure via a suitable choice of their respective parameters. This theory is applied to a two-compartmental structure with an assumed Michaelis-Menten elimination from compartment 1. Models tested for indistinguishability include structures with Michaelis-Menten elimination from compartment 2 in addition to, or instead of, that from compartment 1.