How do you tell if an ad is served up by a big service like DoubleClick?

 

Look at the HTML source code. You can find that in every browser and email client.

 

Below is an actual example from the source code in an email.

 Note that the HTML is cleverly written so you never actually view the code in your browser or email.

 

This is what it looks like if you try to look at it in your regular browser:

That’s because the  tracking device is a small invisible graphic — one pixel-square that connects you to the server that tracks you.

The overall HTML also contains tracking cookies that allow DoubleClick and the online source to follow you and gather your personal data.