Debug.Log(rigidbody.worldCenterOfMass);

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        Debug.Log(rigidbody.worldCenterOfMass);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        Debug.Log(rigidbody.worldCenterOfMass)