How do you locate bad custom content and remove it?

You can use Better Exceptions or Sims 4 Tray Importer to see what's on a Sim. These work quite differently, but their sites will have instructions. You can use Sims 4 Studio to view all your CC, and that will usually reveal this particular error. Finally, you can do vanilla and 50-50 testing of your Mods folder, which is good to learn how to do (especially before the next patch, which is for an EP and will undoubtedly break lots of stuff).