There are some add-ons for Firefox which can simulate data sent to servers to make it seem as if the user agent is browsing on a mobile device, even when they’re not. One of them is Modify Headers addon.
Here is its quick start: http://www.garethhunt.com/modifyheaders/help/quickstart/
Here is the list of iPhone User-Agent: http://enterpriseios.com/wiki/Complete_List_of_iOS_User_Agent_Strings
Here is the other User-Agent profile: http://w3development.de/rdf/uaprof_repository/
Here is iPad UA:
Mozilla/5.0(iPad; U; CPU iPhone OS 3_2 like Mac OS X; en-us)
AppleWebKit/531.21.10 (KHTML, like Gecko) Version/4.0.4 Mobile/7B314
For Google Chrome, you can try out the User-Agent Switcher extension.
Happy testing :)