How to test if a variable exists using JUnit 5?

I would like to test to see if a variable exists in JUnit testing.

I have a class called animal which is an abstract class.

public abstract class Animal {
    private final int age;
    private final int speed;
    
    public Animal (int age,int speed) {
        this.age = age;
        this.speed = speed;
    }
    public static void main(String[] args) {
    }
    @Override 
    public boolean equals(Object anotherObject) {
           if (this == anotherObject) {  
                  return true;  
              }else {
                  return false;
              }
    }
    public abstract Animal[] multiply(int n);
    
    private boolean isFaster(Animal a) {
        if(this.getSpeed() >a.getSpeed()) {
            return true;
        }else {
        return false;
        }
    }
    
    private boolean isOlder(Animal a) {
        if(this.getAge() >a.getAge()) {
            return true;
        }
        return false;
    }
    @Override
    public String toString() {
        return this.getClass()+ "is " + this.getAge() + " years old, is " +this.getSpeed() +" units fast.";
        
    }
    public final int getAge() {
        return age;
    }

    public final int getSpeed() {
        return speed;
    }



}

I would like to test to see if the variable age exists and also if it is private and final. How would I potentially do this in Junit testing?