Code

private void OnLinkCreated(object sender, LinkEventArgs e)
{
	UpdateAnchorPoints(e.Link.Origin);
	UpdateAnchorPoints(e.Link.Destination);
}

private void OnLinkModified(object sender, LinkEventArgs e)
{
	UpdateAnchorPoints(e.Link.Origin);
	UpdateAnchorPoints(e.Link.Destination);
}

private void UpdateAnchorPoints(DiagramNode node)
{
	if (node.AnchorPattern == null)
		return;

	var usedPoints = new Dictionary<int, bool>();

	foreach (DiagramLink link in node.IncomingLinks)
	{
		if (link.DestinationAnchor >= 0)
			usedPoints[link.DestinationAnchor] = true;
	}

	foreach (DiagramLink link in node.OutgoingLinks)
	{
		if (link.OriginAnchor >= 0)
			usedPoints[link.OriginAnchor] = true;
	}

	for (int i = 0; i < node.AnchorPattern.Points.Count; ++i)
	{
		AnchorPoint ap = node.AnchorPattern.Points[i];
		ap.MarkStyle = usedPoints.ContainsKey(i) ? MarkStyle.Circle : MarkStyle.X;
		ap.InvalidateVisual();
	}
}