After some trial and error, I came to the conclusion that the built-in Mobile Emulation in Chrome is the best way to quickly preview Discourse pages on a mobile device.
Sure you can also just append
?mobile_view=1 to an URL, but the emulator has the added benefit of letting you select the screen profile of a specific device.
Edit: I asked Screenfly why Discourse wasn’t working, this is their reply:
Screenfly doesn’t work on every website. Most commonly, this is a security setting on the web server that disallows display inside of frames (like those used in Screenfly).