On some websites - there is a button on the site to switch back to the full version of the site.
On other websites - the only way to get to the full version of the site from a mobile browser is to have the browser "identify" as a desktop browser instead.
Safari doesn't allow you to change what it identifies itself as. Some of the third party browsers in the App Store do.
For example, I use Terra Web Browser (free in the App Store) and it has options to identify as:
iPad Safari Mobile
Mac OS X Safari 5
Internet Explorer 6
Internet Explorer 7
Firefox 3.6