Support Is my bootloader unlocked? How do I install AOKP from here?

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http://androidforums.com – I recently sent my Galaxy Nexus away for repair after some issues with the screen. Since I've had it back, when turning the device on an open padlock symbol displays below the Google logo before the boot graphics. I believe this means the bootloader is unlocked, is that correct? How do I root and install from here? Is there a way to check whether the device is already rooted? I'm using OSX if that makes a difference. (General)