How to check a new udp packet is arrived or not?

void UdpReceivingSocket::OnReceive(int nErrorCode)
{
nRead = Receive(buff, 4096); 
	icount1++;

	switch (nRead)
	{
	case 0:       // Connection was closed.
		Close();
		break;
	case SOCKET_ERROR:
		if (GetLastError() != WSAEWOULDBLOCK)
		{
			AfxMessageBox(L"Error occurred");
			Close();
		}
		break;

	default: // Normal case: Receive() returned the # of bytes received.
		
		buff[nRead] = 0;

CAsyncSocket::OnReceive(nErrorCode);
}

.h file
#pragma once
#include <afxsock.h>

#include "FILE_TRIALDlg.h"
# include "TcpSendSocket.h"


class UdpReceivingSocket :
	public CAsyncSocket
{
	
	void OnReceive(int nErrorCode);
public:

	UdpReceivingSocket();
	~UdpReceivingSocket();
	BYTE buff[4096];
	int nRead;

};