I have smartphone photos on my walls. They look damned good.

Is this person going around asking all of their friends what kind of camera they used to take the photos they have on display? Or are they just sure they can tell from looking?

You absolutely can tell from looking, and you don't even need to be trying. Whenever I show people photos from trips or of my kids or whatnot, they immediately notice the quality ask "You took this on your phone!?" (since I'm showing the photo from my phone library and that would be the default assumption). Sure, people are used to phone photos and they're fine, but even laymen who aren't thinking about judging photo quality immediately notice and appreciate the quality of a real camera photo.