Tracking pixels are just embedded 1x1 images in HTML emails. They're not hidden; they're just stuffed in the rendered HTML. For example, here's one from B&H photo:

    <img src="https://www.bhphotovideo.com/bimages/email-icons/1pximgfortracking.jpg?email=status" />  

Detecting them automatically is probably tricky, but you can avoid the entire problem by not loading external resources in HTML emails (or, better yet, always load the plaintext version of the email.)

Turn off "auto-load images in emails" option in gmail. If a plain-looking email has that banner at top "click here to load images", there is a pixel-tracker in there.

Fastmail shows a banner that indicates image links are included in the email and asks you if you want to load them

I don't load images by default. I use Thunderbird, but I know other clients also have this feature. Definitely worth whitelisting domains that you want to allow images from to avoid triggering the recruiter bots!