In past Fedora releases, the Gnome Tweak Tool can be used to turn on HiDPI for Retina displays. However, I found the option missing in Fedora 27 (at least for the beta version I’m using, but it doesn’t feel like this is an issue of the beta).
While trying workarounds, I found this command to be working: (effective after login again)
$ gsettings set org.gnome.desktop.interface scaling-factor 2
But this command does not have good support for some apps, such as Firefox and even the system file browser. I believe there should be some graphical way to do this, am I right?