there is a property set on nodes that contain form instance data but i don't have access to the preview release source at the moment so i can't tell you exactly what it is. i do remember that it was defined as org.alfresco.web.templating.TemplateService.TT_NAME, so if you have the source you can find the name of the QName there. that, or use the node browser to find the property for form instance data created by forms.
if you do find the name, please post it back to the forum as someone else may find this useful. note that the name has changed since the preview and will not be the same in future releases.