JSLint makes this easy enough to track down. But I wanted something more. I wanted to know who the culprits were. Thus, I whipped up this little script that'll take a list of files and tell you who committed a file with a trailing comma in it. Besides, JSLint, You'll need Rhino and a script to execute commands (named runtime.js). I'll try and post that later.
Like most of the stuff I seem to do lately, this is available as a gist on GitHub. Let me know if you see anything you like.