Form-based visual programming languages, which include commercial spreadsheets and various research systems, have had a substantial impact on end-user computing. Research shows, however, that form-based visual programs often contain faults. We would like to provide at least some of the benefits of formal testing methodologies to the creators of these programs. This paper presents a testing methodology for form-based visual programs. To accommodate the evaluation model used with these programs, and the interactive process by which they are created, our methodology is validation-driven and incremental. To accomodate the user base of these languages, we provide an interface to the methodology that does not require an understanding of testing theory. We discuss our implementation of this methodology and empirical results achieved in its use.