How to run development app in mobile mode

How could I test my apps in mobile mode will in development, without having to post to production, then open on my mobile?

Most browsers will allow you to emulate mobile screen sizes.
Firefox

Alternatively you can connect your mobile device to the same network as your development machine and navigate directly to the URL used when testing from the local browser on your dev machine.
For example:
http://<ip address>:8091/scriptcase/app/samples/menu