How to write a junit test for double linked list's insertion in between operation?

I have a double linked list in which i can perform various functions but I want to write JUnit tests to test those functionalities. I have written unit tests for testing operations like inserting node at end and head, but how to write a test for the insertion in between operation?

Thank you :)

Here is the my linked list and Test class

DoubleLinkedList.java

public class DoubleLinkedList {

  Node head;

  public void add(int data) {
      Node newNode = new Node(data);
      if (head == null) {
        head = newNode;
      } else {
        Node current = head;
        while (current.next != null) {
            current = current.next;
        }
        current.next = newNode;
        newNode.prev = current;
      }
  }

  public void print() {
      Node current = head;
      while (current != null) {
          System.out.print(current.data);
          if (current.next != null)
              System.out.print(" -> ");
          current = current.next;
      }
      System.out.println();
  }

  public int size() {
      int size = 0;
      Node current = head;
      while (current != null) {
          size++;
          current = current.next;
      }
      return size;
  }

  public void addIntoHead(int data) {
      Node newNode = new Node(data);

     if (head == null) {
          head = newNode;
      } else {
          head.prev = newNode;
          newNode.next = head;
          head = newNode;
      }
  }

  public int returnHead() {
      return head.data;
  }

  public void addInMiddle(int prevData, int data) {
      Node current = head;
      while (current != null) {
        if (current.data != prevData) {
            current = current.next;
        } else{
            break;
        }
      }
      Node newNode = new Node(data);
      newNode.next = current.next;
      current.next.prev = newNode;
      newNode.prev = current;
      current.next = newNode;
  }

}

DoubleLinkedListTest.java

import org.junit.Test;
import static org.junit.Assert.*;

public class DoubleLinkedListTest {

  private DoubleLinkedList dll;

  @org.junit.Before
  public void setUp() throws Exception {
      dll = new DoubleLinkedList();
  }

  @Test
  public void shouldBeAbleToCreateANewNode() throws Exception {
      int initialSizeBeforeAdd = dll.size();

      dll.add(1);
      dll.add(2);

      assertEquals(initialSizeBeforeAdd+2,dll.size());
  }

  @Test
  public void shouldAbleToAddIntoHead() throws Exception {
      dll.add(1);
      dll.add(2);

      dll.addIntoHead(12);

      assertEquals(0,dll.returnHead());
  }

  @Test
  public void shouldAbleToAddDataInMiddle() throws Exception {
      dll.add(1);
      dll.add(2);
      dll.add(4);
      int size = dll.size();

      dll.addInMiddle(2,3);

    //what should be the assertion here.
  }
}