You see a simple preview of many ads on Preview tab of the Creative dialog.
Note: Only hosted creatives and secure ad tag formats can be previewed. Non-secure ad tags, VAST, or MRAID creatives will not appear.
The Preview tab shows the ad name, ad ID (a unique identifier for the ad), the ad size, and the clicks since yesterday.
You can preview the ad in its own browser tab by clicking View full image.
Clicks since yesterday shows the number of preview page clicks for the creative. To test your ad's click tracking, click the preview ad and be sure the Clicks since yesterday reflects each of your clicks.
For some unusual ad types it may be necessary to click Refresh clicks to see it increment.