The Chrome browser has decent mobile device emulation, more info on that can be found here:

https://developer.chrome.com/devtools/docs/device-mode

If you're a Mac user, the iOS Simulator that comes with XCode provides a good simulation of iOS devices

https://developer.apple.com/library/ios/documentation/IDEs/Conceptual/iOS_Simulator_Guide/

The ServiceNow mobile UI can also be viewed using Chrome or Firefox   by browsing to <instance>.service-now.com/$m.do.   This won't be an exact match for look and feel, but it will allow you to use the browser's console tools to troubleshoot mobile issues.