Simply add the following at the end of the URL to your planner:
&proof=true
For example, if this is your Online Planner URL:
https://ep70.eventpilotadmin.com/web/planner.php?id=youridhere
simply add &proof=true
https://ep70.eventpilotadmin.com/web/planner.php?id=youridhere&proof=true
Enter the full URL in your browser to load the proof version.