If you find sudden drop from search engine index or disappeared search results, or receive a notice that your site's using cloaking techniques. Its because a malicious party "hacks" a website – penetrate security of a site and insert undesirable content. Sophisticated attackers can camouflage spammy or dangerous content so that it doesn't appear for normal users, and appears only to Googlebot, which could negatively impact your site in Google's search results. Here’s how to tackle this issue:

1. Log in to Webmaster Tools and select your domain that you want to check
2. In the opening page’s Labs section'll contain a feature called ‘Fetch as Googlebot’ at bottom of navigation menu
3. Clicking will open a page – contain a field where you can insert URL to fetch; or just left blank to fetch homepage
4. Just click Fetch and wait a few seconds. After refreshing page, you can see status of fetch request
5. If succeeds, click on "Success" link to get fetched page. Aha! There's the spammy content!

    

[Webmaster Central blog]