how to calculate robot self awareness, differential drive and robot dimensions? [closed]

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I'm looking into slam, rgbslam and autonomous teleop. In some of the packages i explored there is only a variable for base width.

My robot currently has a differential drive setup with the wheels in the front and castors in the back. how would the robot know it is not going to crash the back end into something when making a zero degree turn (each wheel is counter rotating)? is there a package or topic i have to modify and/or subscribe to, to make sure it clears obstacles in the rear.

I'm using a Microsoft kinect (facing forward) to play with slam and mapping, currently. I do not have rotary encoders on the motors yet.

Thanks for your help.

Keith