In this paper we explore how application-users can, in an interactive way, test properties about the state of GUI applications that can be classified as local state transition systems with quiescence. These properties can be added and removed at run-time. It is guaranteed that they are type-correct. We investigate the consequences of such an approach for one particular functional GUI library, Object I/O. The goal is to gain confidence in the quality of interactive applications, and to seek properties that can be proven correct, perhaps using formal proof tools.