Someone posted a site they'd built recently that would send you an email with all the nasty tracking tricks in them, and let you see which your client fell for. I was sad to see gmail choked on most of it.
Make sure to check that the IP addresses are yours. Gmail loads images to their own servers and then serves them to your Gmail UI from there, so trackers shouldn't get your actual IP.
I just tested gmail with emailprivacytester and it didn't fall for any of them (and only fell for the image tags when I clicked a button to allow images in the e-mail to be displayed) so they may have beefed things up in response to that site
