In reply to Nick Halsey.

@Nick: Actually, you can also preview non-published pages today, including drafts.

@Marcus @Jeff: Here is a demo showing how you can use the Customizer to preview mobile resolution for a draft post today, to check how the title wraps:

As Nick notes, with post writing/editing in the Customizer, you'd be able to preview those changes live. But, in fact, the Customize Posts feature plugin (under development) can do this now:

Note that an open issue for the plugin is to change the “Preview” button in the post edit screen with a “Customize & Preview” button so that you wouldn't have to go to the extra step of clicking the Customize link after having clicked the Preview button for a draft post.