You can use several process like
if (panel.Controls.Count > 0)
{
}
or
if (panel.HasControls())
{
}
or
StringBuilder content = new StringBuilder();
StringWriter sWriter = new StringWriter(content);
HtmlTextWriter htmlWriter = new HtmlTextWriter(sWriter);
pnlMyPanel.RenderControl(htmlWriter);
if (htmlWriter.ToString() != "")
{
}