With just a few clicks, you can see what ads may appear on your new web pages, or make an educated decision on whether to add AdSense to your existing site pages. In Firefox, the Adsense Preview Tool is installed in the main toolbar menu.
This is not the official google adsense preview tool, but it exactly has the same ability which is to preview adsense that may shown on any web page.
Latest version available is 1.0.1, this preview tool was designed by Yellow pipe and works only with Mozilla Firefox.
With this tool, you can safely check what ads will be displayed on any online page. It currently works with 8 types of ads which are: Leaderboard (728x90), Banner (468x60), Button (125x125), Skyscraper (120x600), Wide Skyscraper (160x600), Square (250x250), Medium Rectangle (300x250), Large Rectangle (336x280).
You can download this tool here, it's free!