You cannot read the speed for the playback in the main window.
You can subscribe to SmartClient.PlaybackIndication, this will give a message whenever playback speed is changed.
If you are in another window (NOT main) you can read the speed in the view item like this:
private void Button_Click(object sender, System.Windows.RoutedEventArgs e)
{
FQID pbcFQID = this.WindowInformation.PlaybackControllerFQID;
var pbc = ClientControl.Instance.GetPlaybackController(pbcFQID);
double speed= pbc.PlaybackSpeed;
MessageBox.Show("Speed " + speed.ToString());
}