It depends a little on which skin you are using.
In the Larry skin, when you are viewing a message in either preview or full view there is a small down arrow in the bottom right corner of the grey header section that shows the from/to/date etc at the top of the message. Left clicking on this small arrow will display the raw message headers. Clicking it again hides them.
In the Classic skin, when you are viewing a message in either preview or full view there is a small down arrow in the center of the display at the bottom of the grey header section that shows from/to/date etc at the top of the message. Left clicking on this small arrow will display the raw message headers. Clicking it again hides them.
If you want to change which headers are shown by default in that grey section then you need to use a plugin. The show_additional_headers is shipped with Roundcube and is a proof of concept of how to do this.