title: Verify if a PDF is digitally signed with iText
body: <p>As the title says I want to know if a given PDF file is already digitally signed.</p>
  
  <p>I used iText to sign it but I'm not able to know if it is already signed to eventually resign it or perform other actions.</p>
  
  <p>Is there a way to do it simply (possibly using iText)?</p>