Prior to lightning experience enablement in Winter '16, I used to use a handy trick when demonstrating the Salesforce One mobile application to prospective clients. By simply appending '/one/one.app' in the url you could view the application in your browser. e.g. https://ap1.salesforce.com/one/one.app
This was also useful when running training sessions via a large monitor or projector. It would be quite difficult to demonstrate to a room of people using a mobile device.
Simply navigate to the Google Chrome web store at https://chrome.google.com/webstore and search for the UA Spoofer plugin and click 'Add to Chrome'.
Once installed the plugin will display to the right of your browser URL toolbar
You can then click the icon and choose iOS and then subsequently iPhone 6 from the drop down list.
Once you have done this append '/one/one.app' in your url e.g. https://ap1.salesforce.com/one/one.app and hit return. You'll notice that Salesforce will now start to view Salesforce in Salesforce One mode.