using(var doc = PdfDocument.Load(@"e:\0\1GB\signatureFields.pdf", new PdfForms()))
{
    foreach(var field in doc.FormFill.InterForm.Fields)
    {
        if (field.Type != FormFieldTypesEx.Sign)
            continue;

        if (field.Dictionary.ContainsKey("V"))
        {
            //the field contains a signature
            //your code here
        }
    }
}