Alloy is a formal specification language that is widely used to verify software systems. However, while users can verify the properties of a specification with Alloy, it is not so easy for them to validate the specification, that is, to check that the specification is written just as the users intended. Alloy Analyzer, a tool supporting Alloy, has a feature to show concrete instances satisfying specifications that can be help in validation, but it does not control the order in which the instances are shown. Many studies have been conducted on ordering to help users explore instances in structured ways. However, not much prior research has focused on proper ways to explore instances for validating specifications. In this paper, we propose a method to assist users in validating specifications by displaying a set of instances that tend to include problems when their specifications have defects. In particular, the method applies pairwise testing to relations of Alloy specifications. We show effectiveness of the method in experiments using mutation analysis.