Public drafts are available on the official GovTalk web site. Since sample messages are being modified to reflect the current namespace proposals, the validation service has had to be removed until the schemas are in their final locations. By altering the schemaLocation elements in the samples, you can use the W3C service directly. Please read the disclaimer before using this service. The following describes the convention for recently revised schemas.
Older versions use varying conventions.
Normally, this is just the major version since minor versions are upwardly compatible by definition. Pre-release schemas always have a version number starting with 0. Drafts of previously released schemas will have a letter following the current major version. Samples normally have just the schema name with a .xml extension. These will be corrections to previous versions or adding optional information.