Fog Computing is a paradigm aiming to decentralize the Cloud by geographically distributing away computation, storage, network resources and related services. It provides several benefits such as reducing the number of bottlenecks, limiting unwanted data movements, etc. However, managing the size, complexity and heterogeneity of Fog systems to be designed, developed, tested, deployed, and maintained, is challenging and can quickly become costly. According to best practices in software engineering, verification tasks could be performed on system design prior to its actual implementation and deployment. Thus, we propose a generic model-based approach for verifying Fog systems at design time. Named VeriFog, this approach is notably based on a customizable Fog Modeling Language (FML). We experimented with our approach in practice by modeling three use cases, from three different application domains, and by considering three main types of non-functional properties to be verified. In direct collaboration with our industrial partner Smile, the approach and underlying language presented in this paper are necessary steps towards a more global model-based support for the complete life cycle of Fog systems.