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.