1) gztopic hz <topic_name>: will return the average time (over a sliding window) at which a messages are received. Keep in mind that this is not time time at which the message was generated.
2) gztopic view <topic_name>: This is only applicable to Gazebo 1.4+ and only for topics which produce laser or image data. The widget that appears will show the average time at which the data was generated.
3) gzstats: This will show, among other things, the real-time factor. A real-time factor of 1.0 means that Gazebo is running at real-time. A factor <1.0 means that Gazebo is running slower than real-time.