You might want to test your campaigns before showing them to your actual visitors. This is possible if you use this trick:
Campaign settings > When should the popup appear? > Visitors who are browsing one of these pages > add new > equals /?optimonk=true
Using this setting will make your popup to appear ONLY on this page. Go to your site and browse the page added. In this example: mydomain.com/?optimonk=true
The popup will appear on this site so that you can run a test and see it live on your site before your visitors. Since your visitors don’t navigate to this page they will not see it.
Satisfied with the look? Nice! Just don’t forget to delete this setting before launching the campaign.
Alternatively, you can also preview the campaign with your website in a background if you click on
magnifying glass button within Campaign -> Next to Variant button