Also http://detectportal.firefox.com. And http://neverssl.com was set up for this purpose while being a bit easier to remember :)

I remember a while back neverssl.com would happily serve HTTPS requests! Another good alternative is http://httpforever.com/

What gives you confidence example.com won't start serving the HTTPS redirect though? There isn't any reason they wouldn't, and given that browsers are clearly tending towards showing big scary warnings to even accessing something over cleartext, I wouldn't be surprised if they flipped that switch just to avoid confusing noobs.