This bookmarklet analyses the displayed page's HTML source and detects violations of the coding standard. It offers a huge amount of predefined rules and can be extended with custom rules.
Go to HTML_CodeSniffer and get the bookmarklet there.
Activate the bookmarklet by clicking on it. Then inspect the displayed results.
In general, be careful with the displayed results. Automated test tools are very limited and tend to display both false-positive as well as false-negative results.