The definition of Domain-Specific Languages (DSLs) is a recurrent activity in Model-Driven Engineering. However, their construction is many times an ad-hoc process, partly due to the lack of tools enabling a proper engineering of DSLs and promoting domain experts to play an active role. The focus of this paper is on the validation of meta-models for visual DSLs. For this purpose, we propose a language and tool support for describing properties that instances of meta-models should (or should not) meet. Then, our system uses a model finder to produce example models, enriched with a graphical concrete syntax, that confirm or refute the assumptions of the meta-model developer. Our language complements metaBest, a framework for the validation and verification of meta-models that includes two other languages for unit testing and specification-based testing of meta-models. A salient feature of our approach is that it fosters interaction with domain experts by the use, processing and creation of informal drawings constructed in editors liked yED or Dia. We assess the usefulness of the approach in the validation of a DSL for house blueprints, with the participation of 26 4th year computer science students.