This bookmarklet will show you what trackers are firing on a page, where those trackers are included on the Disconnect list of trackers / adtech. This is the list that Firefox's Enhanced Tracking Protection uses for determining which tracking cookies to block.
You may find that as this sort of tracking protection becomes more widely used, that the effectiveness of these scripts/pixels decreases. Depending on what you are using them for, you may wish to remove them to improve page speeds.
Just drag this bookmarklet to your bookmarks bar, and then you can run it on any webpage. It outputs a list of blocked trackers/adtech (both as an alert and in the console).