private function showCentre($doc:DisplayObjectContainer = null, $colour:uint = 0x000000, $radius:Number = 2):void
{
	var dot:Shape = new Shape();
	dot.graphics.beginFill($colour);
	dot.graphics.drawCircle(0, 0, $radius);
	dot.graphics.endFill();
	$doc.addChild(dot);
}

private function showCentre():void
{
	var dot:Shape = new Shape();
	dot.graphics.beginFill(0x000000);
	dot.graphics.drawCircle(0, 0, 2);
	dot.graphics.endFill();
	addChild(dot);
}

A handy little function to dynamically draw a black dot on the stage (or displayObject). This can be useful if you want to see where 0,0 is located.

AS3 Draw a Dot at 0,0 to Show the Centre Of a DisplayObject

center

ActionScript 3