Help detecting when object is grounded

I am unsure how to make it so I can jump only when my character is on the ground I assume it has to do with OnCollisionEnter and OnCollisionExit but am unsure what to put in.

Here is my Script

 using UnityEngine;
 using System.Collections;
 
 public class Runner : MonoBehaviour 
 {
     public float Speed =1;
     public Vector3 JumpHeight = new Vector3();
     public bool IsOnGround = true;
     public float DeathHeight =-10;
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () 
     {
         if (!IsOnGround)
         {
             rigidbody.AddForce (new Vector3 (Speed, 0, 0), ForceMode.Acceleration);
         }
         if (gameObject.transform.position.y < DeathHeight) 
         {
             rigidbody.velocity = new Vector3();
             gameObject.transform.position = new Vector3(1,1,0);
         }
         if (Input.GetMouseButtonDown (0)) 
         {
             IsOnGround = false;
             rigidbody.AddForce(JumpHeight,ForceMode.Impulse);
         }
 
     }
 
     void OnCollisionEnter()
     {
         IsOnGround = true;
     }
 
 
     void OnCollisionStay()
     {
         IsOnGround = true;
     }
 
 
     void OnCollisionExit()
     {
         IsOnGround = false;
     }

I am unsure what to put in the script to make it so the jump only works when I am on an object. (This is for an endless runner if that helps at all)