[SOLVED] pcl_ros: how to find cloud frame

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Dear all, I am using pcl_ros package, with pcl 1.6 and ROS groovy. I am subscribing to a PointCloud2 topic, using a callback like this:

void MainWindow::xtionCloudCallback(const pcl::PointCloud<pcl::pointxyzrgb>::ConstPtr& cloud)

is it possible to get the cloud frame? if I transform the cloud using a TransformListener tf, e.g.:

pcl_ros::transformPointCloud("/map", *cloud, *tcloud, tf);

this function is able to retrieve the cloud frame in some way, how do I get it?