new TextRange(Document.ContentStart, Document.ContentEnd).IsEmpty

var start = rtb.Document.ContentStart;
var end = rtb.Document.ContentEnd;
int difference = start.GetOffsetToPosition(end);

public bool IsRichTextBoxEmpty(RichTextBox rtb)
{
    if (rtb.Document.Blocks.Count == 0) return true;
    TextPointer startPointer = rtb.Document.ContentStart.GetNextInsertionPosition(LogicalDirection.Forward);
    TextPointer endPointer = rtb.Document.ContentEnd.GetNextInsertionPosition(LogicalDirection.Backward);
    return startPointer.CompareTo(endPointer) == 0;
}

bool IsEmpty(Document document)
{
    string text = new TextRange(Document.ContentStart, Document.ContentEnd).Text;
    if (string.IsNullOrWhiteSpace(text) == false)
        return false;
    else
    {
        if (document.Blocks.OfType<BlockUIContainer>()
            .Select(c => c.Child).OfType<Image>()
            .Any())
        return false;
    }
    return true;
}

string text = new TextRange(Document.ContentStart, Document.ContentEnd).Text;
return !String.IsNullOrWhiteSpace(text);

Private Function RichTextBoxIsEmpty(BYVAL rtb As RichTextBox) As Boolean

    Dim ReturnCode As Boolean = True

    Dim text As String = New TextRange(rtb.Document.ContentStart, rtb.Document.ContentEnd).Text

    If String.IsNullOrWhiteSpace(text) Then

        For Each block As Block In rtb.Document.Blocks

            'check for an image
            If TypeOf block Is Paragraph Then
                Dim paragraph As Paragraph = DirectCast(block, Paragraph)
                For Each inline As Inline In paragraph.Inlines
                    If TypeOf inline Is InlineUIContainer Then
                        Dim uiContainer As InlineUIContainer = DirectCast(inline, InlineUIContainer)
                        If TypeOf uiContainer.Child Is Image Then
                            ReturnCode = False
                            Exit For
                        End If
                    End If
                Next
            End If

            ' Check for a table
            If TypeOf block Is Table Then
                ReturnCode = False
                Exit For
            End If

        Next

    Else

        ReturnCode = False

    End If

    Return ReturnCode

End Function

var rtf = GetRtfText();

var start = Document.ContentStart;
var end = Document.ContentEnd;
var difference = start.GetOffsetToPosition(end);

HasText = difference > 4 || rtf.Length > 350;


public string GetRtfText()
{
  var tr = new TextRange(Document.ContentStart, Document.ContentEnd);
  using (var ms = new MemoryStream())
  {
    tr.Save(ms, DataFormats.Rtf);
    return Encoding.Default.GetString(ms.ToArray());
  }
}