Firstly, check your extensions to see what marketing pixel-related extensions have been installed, e.g. Meta Pixel, Google Tag, etc. That will give you a clue as to what to look for.
Then... comes the hard part: finding the Rules that contain actions that send the marketing pixels. If you found extensions earlier, then you might be able to find the related actions in one or more of your Rules.
Also, marketing pixels could be added via custom code actions. Unfortunately, there's no easy way to find them other than by inspecting the JS file of your environment. If you have a small number of Rules, you could also take the time to open them one by one and see if any of them contain custom code actions that might be firing marketing pixels.
Finally, you can also try using Tags' debug mode in your browser to get some clues. Open your website. Then, open your browser console and run "_satellite.setDebug(true)" (without the quote marks). This puts your Tags environment in debug mode. Now reload your page. Your browser console will fill up with several log messages. Some of them might be about marketing pixels. Use those messages to hunt down the actions in your Rules that are firing those marketing pixels.
The best solution for me is to take the adobe launch library endpoint, remove the .min and only keep .js, then load it in the browser.
After this use the search functionality of your browser to search for keywords and you should see all code related to your marketing pixel.
Also you would need to know the code for the marketing pixel and if an extension has been used...
So do search for lets say "facebook" but also the facebook object which is "fbq"
Hope this helps