This is a simple script that automatically checks all citations in an article to see if the link 404s. If it does, it will be outlined with a dashed red line.
This tool can have false positives. It outlines links when auto citation fails, however auto citation could fail for many reasons (for example, on facebook urls). I am working to reduce the number of false positives, but you should use this as a quick heuristic, not as religion.