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