Joomla Plug-ins - System
Is Robot

bfisrobot
Free

Plugin for detecting when site is visited by a robot.
Defines constant IS_CRAWLER with name of crawler, otherwise null.
Make sure it runs before any other plugins which require this functionality!

Uses Joomla's builtin robot functionality to which it appends its own predefined list of robots.
You can add any others you encounter to the plug-in configuration screen.

Note that this plugin does not block the robots, it simply identifies the presence of a robot.
Blocking of robots, if needed, is best done before they get as far as the Joomla site 'index.php' file!