When document is downloaded from server it contains various metadata information e.g. Id or current etag. This information is stored in session and available for each entity using GetMetadataFor method from Advanced session operations.
RavenJObject GetMetadataFor<T>(T instance);
Instance of an entity for which metadata will be returned.
Returns entity metadata. If the instance is transient it will load document from server and attach entity and its metadata to session.