Good news: the Xbox One has game demos! Bad news: they're kind of annoying to find.
You can't see the free versions of each game in the Xbox One store itself—and there aren't all that many available yet (though Microsoft's Larry Hryb promises to keep us updated)—so if you want to find demos for the likes of FIFA and NBA Live, you'll have to search yourself. Two easy steps:
1) Say "Xbox, Bing" or select the Bing search bar on the main menu.
2) Say or type "demos." Here's what you'll see:
Note: these instructions also apply if you're looking to find the album Independiente + Demos by Ricardo Arjona, which includes tracks like Mi novia se me está poniendo vieja and Lo que está bien está mal.