Co-simulation of modular process plants combines partial models for each Process Equipment Assembly (PEA) from different manufacturers. Considering the manufacturer's intellectual property (IP), the co-simulation of the partial models must be realized via a standardized interface using Functional Mock-up Units (FMU). This approach also requires a flexible approach to model quality assessment. In this paper, a workflow for the quality assurance of modularized simulations models for co-simulation is presented via the example of a modular electrolysis plant. The first step is an analysis of the required partial models to derive requirement specifications, the resulting quality models, and the test specifications. These specifications lay the groundwork for the test implementation for the modular electrolysis plant. This use case addresses eighteen partial models of varying modeling depth, including models provided solely as a FMU.