You need to create a class that implements the interface com.badlogic.gdx.physics.box2d.RayCastCallback so that your class has a method:
float reportRayFixture(Fixture fixture, Vector2 point, Vector2 normal, float fraction)
Then call this method:
World.rayCast(RayCastCallback callback, Vector2 point1, Vector2 point2)
and pass in an instance of your class as the first parameter.
Your implementation of the reportRayFixture method will then be called by Box2d. Your implementation should save the Fixture that is passed into it, this is how you get the fixture data.