How can they trace you?
For example, many websites use “Google fonts” as external resources. Other web sites embed external videos or maps inside a page.
How can you check this?
On Mozilla Firefox, you can active the Firefox Web Console and view all the external network resources accessed by that web page by clicking Ctrl + Shift + K or by clicking the three-lines menu.
The following screenshots show you how to access the Firefox Web Console from the three-lines menu.