How can I know if a website uses Facebook Pixel:
There are two ways you can check if the website uses Facebook Pixel.
1.If the website is NOT using GTM(Google Tag Manager), you can use view page source option to check if the website uses Facebok Pixel. Just right click anywhere on the browser to show and select the view page source option. Then look and search for the <!-- Facebook Pixel Code --> section.
2. If the website is using GTM to install all the pixels and conversion tracking. The only way you can check if the website uses Facebook Pixel is through Google Chrome "Facebook Pixel Helper" add on.