Jay,
To check How the content will look in between the breakpoints, you can use the Project -> Preview option. It will show a slider in the browser. Moving the slider will change the preview widths and the content appears according to the width set in the slider. The Preview will give you a fair idea on how the spacial relationships and size of the objects will change between the breakpoints. You can then re-design the slide (if required) by changing the Position parameters to suit your needs.
Tamish