Check if float is increasing/decreasing

Hello, I am in the process of recreating flappy bird, I am fairly new to C# and Unity. I am stuck at jumping, I want to create a float, which will be the bird's altitude, then I want to check if the float is increasing/decreasing. If it is increasing or decreasing, then I want the state that to a boolean named isFlapping. Sorry, I don't have any code that is worthy yet, therefore can't show any code so far. Let's take this code for an example:

 using UnityEngine;
 using System.Collections;
 
 public class BirdPhysics : MonoBehaviour {
 
     public float jumpHeight = 8;
     public bool isFlapping = false;        //Check if bird is jumping
     
     void Start () {
         
     }
 
     void Jump(){
         rigidbody2D.AddForce(new Vector2(0f,jumpHeight));
     }
 
     void Update () {
         if(Input.GetKeyDown("up")){
             Jump ();
         }
     }
 }