At any time when editing your course, you can play it and launch the course in a new tab or window, allowing you to display and test the course as if you're a student user on the Learning Management System you plan to host it on. You will need to save any changes made (both in navigation and in content) before they will show in the player.
(Click image to enlarge)
If keeping the player open while editing and saving changes, the course should update as soon as you refresh or navigate to previous/next pages. However, if updating navigation or inserting brand new images to the course, you'll need to relaunch the player before the changes will show.
In the Catalogue Landing Page, you can run the player by clicking the Play button in the Actions panel. In the Course Landing Page, you can run the player by clicking the large green Play button in the top right corner of the screen.
Many of the settings in the editor's Settings tab will come into play here - as such, said tab will often resemble the selected player layout to give you an idea on which options affect which areas (for instance, all settings related to the header in the Coursesuite layout are placed in a area resembling the header as it will appear in the player).
By the way, don't be alarmed if when you open the course builder, it looks like it's collapsed in on itself:
It's working fine - the player just needs to be resized. See Additonal Options for how to do that.