Trig, speed and direction

1

using trig, when moving an object based on its current rotation, how does one “detect” if the object is currently moving to** its own** left or right direction?
example:
right side up: left moves the car to its left, which is left.
upside down(180 rotation): left moves it the car to its left, which is right.
based on movement alone, how can i say its moving to its own oriented left or its right?

here is an example:
paste into frame 1 of a new fla.


addEventListener(Event.ENTER_FRAME,  go);
stage.addEventListener(KeyboardEvent.KEY_DOWN,keysDown);
stage.addEventListener(KeyboardEvent.KEY_UP,keysUp);

var  vx:Number=0;
var vy:Number=0;
var radians:Number;
var  speed:Number=0;
var left:Boolean;
var right:Boolean;

//create  "car"
var car:Sprite = new Sprite();
car.graphics.lineStyle(3,0x00ff00);
car.graphics.beginFill(0x0000FF);
car.graphics.drawRect(0,0,70,25);
car.graphics.endFill();
car.graphics.drawCircle(10,  30, 5);
car.graphics.drawCircle(60, 30, 5);
addChild(car);

//create  "instructions"
var txt:TextField = new TextField();  
txt.width =  250;  
txt.x = 25;  
txt.y = 300;  
txt.text = "left arrow :  LEFT";
txt.appendText("
"+"Right arrow: RIGHT");
txt.appendText("
"+"Up  arrow:  stop and rotation+=10");
addChild(txt);

//game Loop
function  go(e:Event) {

    if (left==true) {
        speed=-1;
    }  else if (right==true) {
        speed=1;
    } else {
         speed=0;
    }

    radians=car.rotation*Math.PI/180;
     vx+=speed*Math.cos(radians);
    vy-=speed*Math.sin(radians);
     car.x+=vx;
    car.y-=vy;

}
function  keysDown(e:KeyboardEvent) {
    if (e.keyCode==37) {
         left=true;
        right=false;
    }
    if (e.keyCode==39) {
         right=true;
        left=false;
    }
    if  (e.keyCode==38) {
        car.rotation+=10;
         radians=car.rotation*Math.PI/180;
        //var  currentSpeed:Number=Math.sqrt(vx*vx+vy*vy);
         //vx=currentSpeed*Math.cos(radians);
        //vy=-  currentSpeed*Math.sin(radians);
        vx=0;
        vy=0;
     }
}
function keysUp(e:KeyboardEvent) {

    if  (e.keyCode==37) {
        left=false;
    }
    if  (e.keyCode==39) {
        right=false;
    }
}