This page tests whether the RTA label, when included exaclty as described on their How To page, is respected.
If you are seeing this page, your current setup does not block content labelled in this way.
The <meta name="RATING" content="RTA-5042-1996-1400-1577-RTA"> tag is commonly used by adult websites to mark themselves as such. Since rtalabel.org also tells people to include an image of the RTA logo on their site, it is often visibly apparent if a website is using this tag by checking the footer of the page for the RTA logo.