There are a few ways.
1) use Firefox browser and user agent switcher addon 0.7.3 You can then select an iphone agent and refresh the browser and you should see the iphone site. You also need this agent switcher if you are going to use
www.testiphone.com and view the site through it.
2) download and install iBBdemo2 from
labs.blackbaud.com/NetCommunity/article?artid=662
3) Use address
iphone.sitename.com instead of
www.sitename.com
4) Use the extensions > plugin manager > mobile - always >
parameters > plugin parameters > mobile markup
and choose iphone from the drop down.
enable and save. This makes the site appear as iphone for EVERY visitor. Which you may not want.
5) Internet Explorer also has a user agent switcher (same a 1 above).
If you switch backwards and forwards using mobile view / standard view. It sometimes gets "stuck". You can sometimes overcome this by clearing all cookies (session and domain) and clearing any cache in the browser.