The tool can be found for all English users under the Labs section of Google Webmaster Tools.
Since the launch of Instant Previews, Google has continued to improve them but some webmasters had issues with Google showing nice previews. So this tool is designed to help webmasters diagnose these problems and improve their instant previews within Google.
You enter the URL of the page you want an Instant Preview preview of and Google will show you the preview. Plus, Google will offer automated feedback on problems Google's system can detect such as missing or roboted resources.
Let's compare the live version versus what I see in Webmaster Tools, which Google admits might be slightly different.
Webmaster Tools Preview:
There are two threads talking about this new feature, one at Google Webmaster Help and the other at WebmasterWorld. One person said he "ran the test 3 times for the same page and I got 3 slightly different results, every time 1-3 images did not make it down the tubes, not the same ones every time either." Another person said "when I compare my site to the instant preview google is showing 35 errors but the instant preview is working fine."
I am sure the "labs" product will improve over time and I am happy it is there, even with some limitations.