If this tool fails to display ads for a particular keyword or web domain, chances are that it is a restricted keyword or the domain is not part of the AdSense program.
Google Adsense Preview Tool will display contextual and geo-targeted Google AdSense ads for any website (e.g., cnn.com) or keywords (e.g., airline tickets) based on standard AdSense targeting and filtering.
It uses your default geographic location (based on your IP address) for geo-targeting but you can also choose a different country from the dropdown.
Unlike the Official Adsense registry add-on that works only with IE browser, this online Adsense Preview Tool works across all web browsers including Google Chrome, IE, Firefox, Opera and Safari.
The AdSense Preview tool supports text ads (CPC), image ads, rich media ads (Flash) and Ad Link units in all standard ad formats including Leaderboard (728×90), Skyscraper (120×600 & 160×600), Rectangle (336×280 & 300×250) and Square (200×200)
The internal code uses random Adsense Publisher IDs with ad_test set to “ON” and therefore the Adwords advertisers will be not be charged for any clicks or ad impressions generated through this tool. Google AdSense Sandbox – Preview Tool for Google Ads