In Gnome keyboard settings, there's a window in which you can see the layout, a keyboard with buttons and characters on them like this:
In XFCE (Xubuntu 12.04) there's no such program.
What can I install to see it?
In XFCE it is possible to install Gnome Panel applications. If native panel application is not available in XFCE, you can consider adding the Gnome Panel Application that does the same job.