Click on the Security Issues tab on the side bar of GWT.
The Security Issues pages lets you know of any known security issues with your site. If you visit your website and get a malware warning from your browser, it is likely that your website is referencing code or content from another website that has malware.
If your site has been hacked or compromised, the search results of your website on Google will show up with “This site may harm your computer.” Or “This site may be hacked.” This will avoid people from visiting your website. It is important to check the Security Issues tab regularly to see whether Google has marked your site as being hacked.
There are numerous things that can cause your site to be compromised such as: server configuration, SQL injection, code injection, error template, binary malware, content injection and URL injection. One way to address these issues is by locating the problematic content and removing it.