Breakpoint Tester Is a Bookmarklet Which Scans Website Stylsheets (Same Domain) To Find User Defined Media Query Breakpoints.
Basically, It Lets You Check Responsive Designs By Breakpoints Rather Than Fixed Device Widths.
Drag above button to the browser's bookmark bar
*Note : Breakpoint Tester is still in it's initial phase. You might find some bugs detecting breakpoints. Feel free to provide us feedback.