Three.js THREE.Matrix4 find the face of cube showed

Three.js THREE.Matrix4 find the face of cube showed

I'm doing tests with Three.js, i create a cube obj and I found the
location of the cube
scene.updateMatrixWorld(true);
var position = new THREE.Vector3();
position.getPositionFromMatrix( cube.matrixWorld );
console.log(cube.matrixWorld);
console.log(position);
alert(position.x + ',' + position.y + ',' + position.z);
but i need find the face of cube showed.
thank you in advance