When testing the promotion it is a good idea to be aware of the different ways you can perform a test:
View the promotion in a standard browser session
View the promotion in a separate browser tab
View your promotion in a "Private/Incognito" browser session
Default behavior
It depends on how you have configured the promotion. By default Discount Ninja will avoid displaying the pop-up multiple times during the same browser session. This avoids frustration for your visitors.
About "once per session" and opening new tabs
When testing, you may be surprised that your pop-up doesn't show when you refresh the page (F5). This happens when you configure your pop-up to be shown "once per session". It happens because Discount Ninja detects, after the page is refreshed, that you are within the same browser session. Simply open a separate tab (CTRL+T in most browsers) and copy the URL to start a new browser session.
About "once per device" and Private/Incognito browser sessions
If you have configured the pop-up to be shown "once per device", opening the same promotion on a new tab will not show the pop-up. Discount Ninja will detect that it has already been shown on this device. How can you work around that? Open a different browser (e.g. Firefox instead of Chrome) or open a Private/Incognito browser session to simulate a new visitor coming from a different device.
Conclusion
If you want to see the experience for a new visitor, simply open a Private/Incognito browser session to test your promotion.