Use the FeedValidator tool to verify that your feed data file matches the specification defined in this document.
Use the ScheduleViewer application to see your feed data represented on a map. This is not representative of how your data will look in other applications; it is a basic tool for testing. Examine routes and schedules to ensure that the data feed correctly represents your system.
The Chouette project (managed by the French Agency for Multimodal Information and Ticketing) contains validation functionality and can translate between various formats.
GTFS-realtime Validator - A tool created by the Center for Urban Transportation at the University of South Florida to verify that your real-time feed data correctly matches your GTFS dataset and contains all required information. You can run the software yourself, or you can try the version hosted by USF.