That's getting harder and harder as people add HSTS.
There's still neverssl.com, but with the most popular pages using HSTS like Google Facebook and Reddit, captive portal detection is essential for your average user.
Although I wish the IETF would make a standard for doing this at the network level as part of DHCP rather than the current ridiculousness we have. Captive portals are just the buggiest shit.
For anyone wondering: Just try and open literally any http page (note: no s). I use groklaw.net.