How to locate origin of world frame in Rviz ?

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As far as RViz has a world frame, it's probably at 0, 0, 0. That would be in the centre of the grid you see rendered in your screenshot.

You can visualise it by adding an Axes display to your configuration.

But note:

1. the name world has no special significance to RViz
2. the 'root' of your transform tree will be whatever you make it. It will depend on the transform broadcasters that you have running. There is no 'always present' frame called world

If you're using TF (and I believe you are), it would probably be more informative to add a TF display to your configuration. That would immediately show you how things (ie: frames) are related.