A common technique is that somewhere in the email you'll see something like:

<img src="https://example.com/cd726f02-d2f4-4c0e-a717-e69044180c59.gif" height="1" width="1">

The image filename is a UUID. The UUID is of course unique for each email sent, but the Web server is configured to serve the same image for any given UUID (after recording the UUID as an "open" into a database).

There isn't a way for an email client to be certain that the image is or isn't a tracking pixel.