It's kind of weird, because people I know would have opinions about KDE vs Gnome and they didn't say anything exactly wrong, but I've never heard anyone ever talk about it like that.
You can't really guess which distro someone is using just by glancing over their screen for a second.
You can tell which DE they are using but not distro, except in the case of Unity and Pantheon, those are mostly used with their respective distros. People don't usually have output of cat /etc/os-release on the screen easily visible for people passing by to see.
There isn't that much visible difference between distros if you aren't working as root.
that would be something credible, but hey they are getting really close!