How To Check isGrounded Or Not PLEASE HELP

I have a C Sharp script and I got it to jump. But what I found out is it double jumps, triple jumps, there is no limit to how much you jump! Can you help me?

 using UnityEngine;
 using System.Collections;
 
 public class PlayerJump : MonoBehaviour {
     
     public float jumpForce = 800f;
 
     private bool jumping = false;
     private Rigidbody2D rb;
 
     void Start () {
         rb = GetComponent<Rigidbody2D> ();
     }

 
     void Update()
     {
         if (Input.GetKeyDown (KeyCode.Space) && !jumping) 
         {
             jumping = true;
         }
     }
     
     void FixedUpdate () 
     {
         if (jumping) 
         {
             rb.AddForce(new Vector2(0,jumpForce));
             jumping = false;
         }
     }
 }

I tried to add the variable "jumping" to stop this but it didn't help.