Endpointing devices, planarizing machines with endpointing devices, and methods for endpointing mechanical and/or chemical-mechanical planarization of microelectronic substrate assemblies. One endpointing apparatus in accordance with the invention includes a primary support member for supporting either a polishing pad or a substrate assembly, and a secondary support member coupled to the primary support member. The primary support member is movable with respect to the secondary support member in a lateral motion at least generally parallel to the planarizing plane in correspondence to the drag forces between the substrate assembly and the polishing pad. The endpointing apparatus also includes a force detector attached to at least one of the primary and secondary support members at a force detector site that can have a contact surface transverse to the planarizing plane. The force detector measures lateral forces between the primary support member and the secondary support member in response to drag forces between the substrate assembly and the polishing pad. In operation, the endpoint of CMP processing is detected when the measure lateral force is equal to a predetermined endpoint force for a particular CMP application.