Incognito basically just means "doesn't show up in my history". There are many, many, ways to track users without needing cookies.

> doesn't show up in my history

AND drop all cookies from all domains.

> There are many, many, ways

There probably are. I haven't ever seen it work though. If I get into incognito, my ads show something different my normal profile.

In theory, they can track you from your OS/browser combination (and many more variables), but is there a way to test it?

It isn't theoretical. Google is currently facing a lawsuit (Brown et al v Google LLC et al) for tracking people who assumed Chrome's incognito meant Google would stop tracking them. [0]

[0] https://www.gizmodo.com.au/2020/06/google-facing-us5-7-billi...

That's a hilarious premise for a lawsuit, because that's never what incognito mode meant.

The EFF has a great tool demonstrating the method: https://panopticlick.eff.org/

It’s not theoretical; it’s used in practice today.

Font enumeration is the big one.