The FB Pixel Helper is a troubleshooting tool to help you with your FB implementation.
Download for Chrome: https://chrome.google.com/webstore/detail/fb-pixel-helper/fdgfkebogiimcoedlicjlajpkdmockpc?hl=en
Download for Firefox: https://developers.facebook.com/docs/ads-for-websites/pixel-troubleshooting
Once installed, it works in the background to automatically review the websites you browse for code that looks like a FB marketing pixel. When a website has a FB pixel installed the </> icon turns blue with a small green badge indicating the number of pixels found.
If the pixel does not load, the number of errors found appear in red.