Joomla Plug-ins - System
Is Mobile

bfismobile
Free

Plugin for detecting when site is visited by a mobile.
Defines constant IS_MOBILE which is set to true, otherwise it is undefined.
Make sure it runs before any other plugins which require this functionality!

First checks for the existence of PHP configuration variable 'is_robot' (if defined in your server configuration, php.ini or .htacess). Allows you to provide a custom subdomain for mobiles (e.g. 'mydomain.com' and 'm.mydomain.com') without the need for further checks (this is optional).

If not set then uses Joomla's builtin mobile functionality to detect mobile user agents.
You can add further user agents using the plug-in configuration screen.

If required, when a mobile is detected, you can automatically redirect to the URI for mobile version of site.