signedXml.LoadXml((XmlElement)nodeList[0]);
...
foreach (KeyInfoClause kl in signedXml.KeyInfo)
{
    KeyInfoX509Data data = kl as KeyInfoX509Data;
    if (data != null)
    {
        foreach( X509Certificate2 cert in data.Certificates )
            Console.WriteLine(cert.Subject);
    }
}