How do I force a serial port write method to wait for the line to clear before sending its data?

for (int attempt = 0; attempt < 3; attempt++)
{
    try
    {
        serialPort.Write(labelData);
        break;
    }
    catch (TimeoutException ex)
    {
        // Log info or display info based on ex.Message
        Thread.Sleep(3000);
    }
}

serialPort.Handshake = Handshake.RequestToSendXOnXOff;
serialPort.WriteTimeout = 10000; // Could use a lower value here.

serialPort.Write(labelData);