How do I check whether an element is already bound to an event?

const btns = document.getElementsByClassName('reply-btn');

for (let i = 0; i < btns.length; i++) {    
  btns[i].addEventListener('click', showCommentContentAsPreview);
}


function showCommentContentAsPreview(e) {
  console.log('showCommentContentAsPreview()');
  
  // CHECK IF THIS BUTTON ALREADY BINDED !!!
  const previewDiv = document.getElementById('preview');
  const commentId = e.target.getAttribute('data-comment-id')
  const commentDiv = document.getElementById('comment-' + commentId);
  const commentText = commentDiv.querySelector('p').innerText
  const closeReplyBtn = previewDiv.querySelector('button');
  const previewContent  = previewDiv.querySelector('.preview-content');
  
  // set to preview
  previewContent.innerText = commentText;
  
  // show reply close button  
  closeReplyBtn.classList.remove('hidden');
  
  // bind EventListener to "reply close button"
  closeReplyBtn.addEventListener('click', closeReply)
  
  function closeReply() {
    console.log('bind to btn');
    previewContent.innerText = '';
    this.removeEventListener('click', closeReply);
    closeReplyBtn.classList.add('hidden');
  }
}

.hidden {
  display: none;
}

.comment {
  border-bottom: 1px solid #000;
  padding: 5px;
}


.preview {
  background-color: #ccc;
  padding: 20px;
  margin-top: 20px;
}

<div>
  <!-- comment list -->
  <div id="comment-1" class="comment">
    <p>Comment Content 1</p>
    <button class="reply-btn" data-comment-id="1">reply</button>
  </div>
  
  <div id="comment-2"  class="comment">
    <p>Comment Content 2</p>
    <button  class="reply-btn" data-comment-id="2">reply</button>
  </div>  
</div>

<!-- output -->
<div>
  <div id="preview" class="preview">
    <div class="preview-content"></div>
    <button class="hidden">Close Preview</button>
  </div>
</div>