Physically motivated models of electromechanicalmotion systems enable model-based control theory and facilitatesystem interpretation. Unfortunately, the effort of modellingrestricts the usage of model-based methods in many applications.Some approaches to automatically generate models frommeasurements choose the best model based on minimizing theresidual. These model selection attempts are limited due toambiguities in reconstructing the internal structure from theinput-output behaviour because usually motion systems haveonly one actuator and one sensor. Often, it is unknown if theresulting model is unique or if other models with differentstructure would fit equally well. The set of candidate modelsshould be designed to contain only distinguishable models butambiguities are often unknown to the experimenter. In thispaper distinguishability is investigated systematically for a classof multiple mass models representing servo positioning systems.In the analysis a new criterion for indistinguishability is used.The benefit of additional, structural sensors on distinguishabilityof models is demonstrated which suggests to mount themtemporarily for the commissioning phase in order to facilitatethe model selection. It turns out that the best results can beachieved if synergies among sensor signals are utilized.© 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, includingreprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.