Box2d Single Collision check using cocos2dx 3.0a

cocos2d-x C++
#1

suppose I create two box2d bodies with their shapes and definition, also set their position, now at any point of time I want to check if they overlap or collide, how can I do this, please help!

i create a body like this

b2BodyDef ballBodyDef;
      b2Body *ballBody;
      b2PolygonShape ballShape;
      b2FixtureDef ballShapeDef;

ballBodyDef.type = b2_dynamicBody;
    ballBodyDef.position.Set(ball->getPosition().x/PTM_RATIO, 
        ball->getPosition().y/PTM_RATIO);
    ballBodyDef.userData = ball;
     ballBody= _world->CreateBody(&ballBodyDef);



    ballShape.SetAsBox(ball->getContentSize().width/PTM_RATIO/2,
                         ball->getContentSize().height/PTM_RATIO/2);

    ballShapeDef.shape = &ballShape;
    ballShapeDef.density = 10.0;
    ballShapeDef.isSensor = true;
    ballBody->CreateFixture(&ballShapeDef);