Scenekit: How to check in which direction a node is facing toward camera

Question

I am creating a 3D modeling module in my app. I am rotating the Node with following code:

-(void)handlePan:(UIPanGestureRecognizer *)gesture{

NSString *nodeName = nil;

for (SCNNode *node in _sceneView.scene.rootNode.childNodes) {
    if(node.name != nil){
        nodeName = node.name;
        break;
    }
}

SCNView *scnView = _sceneView;

SCNNode *humanNode = [scnView.scene.rootNode childNodeWithName:nodeName recursively:YES];

// get pan direction
CGPoint velocity = [gesture velocityInView:self.view];

if (panDirection ==nil) {

    panDirection = [self getPanDirection:velocity];
}
// if selected brick
if (gesture.state == UIGestureRecognizerStateBegan) {
    lastAngle = 0.0;
}

CGPoint translation = [gesture translationInView:self.view];

float anglePan = ([panDirection isEqualToString:@"horizontal"]) ?  [self deg2rad:translation.x] : [self deg2rad:translation.y];
float x = ([panDirection isEqualToString:@"vertical"]) ? 1 : 0.0;
float y = ([panDirection isEqualToString:@"horizontal"]) ?  1 : 0.0;

// calculate the angle change from last call
float fraction = anglePan - lastAngle;
lastAngle = anglePan;

NSLog(@"%f",fraction);

// perform rotation by difference to last angle
humanNode.transform = SCNMatrix4Mult(humanNode.transform, SCNMatrix4MakeRotation(fraction, x, y, 0.0));}

I want to know if the Model is facing as front toward the camera or as back from the camera. How can I accomplish this?