You can use Facebook Pixel Helper to find out whether a pixel is already installed on your website.
According to Facebook:
The Facebook pixel is an analytics tool that allows you to measure the effectiveness of your advertising by understanding the actions people take on your website.
Facebook Pixel Helper is a troubleshooting tool that helps you find out there is a Facebook pixel installed on a website, check for errors, and understand the data that’s coming from your Facebook pixel.
Facebook Pixel Helper only works with Chrome, however. It is a Chrome plugin developed by Facebook.
Start by opening a new Chrome browser.
In the address bar, type in https://chrome.google.com/webstore/category/extensions
In the search box, type in Facebook Pixel Helper and search.
Locate Facebook Pixel Helper in the search results.
Click Add to Chrome.
A confirmation popup will appear. If you consent, click Add extension.
After a few seconds, a confirmation message will appear.
You will find a pixel helper icon appears next to the address bar.
Now that your Chrome is equipped with Facebook Pixel Helper, you are ready to test your website.
Using Chrome, with pixel helper icon visible, go to your website.
While on your website, if the icon remains gray and looks inactive, it means you do not have Facebook pixel installed on your website.
If the icon lits up with a number in green, it means your website already has pixel installed. (The number indicates the number of “events” is being tracked.)
You can remove the plugin anytime by right-click on the icon and click Remove from Chrome…
Read more:
https://developers.facebook.com/docs/facebook-pixel/support/pixel-helper