Processes for manufacture and assembly of implantable medical devices are described. In particular, techniques are provided for nondestructive electrical isolation assessment of feedthrough assemblies of the implantable medical devices. The feedthrough assemblies may include an insulating structure, a plurality of terminal pins extending through the insulator and a ferrule having an inner lumen into which the insulating structure is disposed. One or more insulating seals may be disposed at the interface of the ferrule-to-insulating structure and/or the terminal pin-to-insulating structure. The electrical isolation assessments may be based on the dielectric properties of the components of the feedthrough assemblies, such as the insulating structure.