var forms = new PdfForms();
var document = PdfDocument.Load(filename, forms);

var disiredControl = yourField.Controls[0];

for (int i = 0; i < document.Pages.Count; i++)
{
    var controls = forms.InterForm.GetPageControls(document.Pages[i]);

    foreach (var ctrl in controls)
    {
        if(desiredControl.Dictionary.Handle == ctrl.Dictionary.Handle)
        {
            //Got it!
            break;
        }
    }
}