This is a Chrome extension that will resize your browser window to the break points in the code of the current site you are viewing. Those sites without breakpoints show a blank window in the extension.
Responsive Inspector is helpful to see all of the breakpoints a particular site uses. Some have several breakpoints, other sites have just a few. It's a good learning tool.
It's a wee bit buggy sometimes, but it's being updated regularly.
If this link does not work, just search for "Responsive Inspector" in the Chrome browser Extensions.
Group access extension:
JUGA is Joomla component that allows you additional control over what content is available to which users. It is NOT A HACK (like JACL); JUGA installs and uninstalls *easily* as a component. To activate/de-activate it, install and publish/unpublish the JUGA Mambot. After installing JUGA, merely synchronize it's table with your site's components and content. Then, define which items each group has access to. After assigning users to JUGA groups, activate the Mambot and voila!