The Available property of the System.Net.Sockets class will tell you how much data is available to read.

With UDP sockets, one thing to remember is that Socket.Available will give the total size of all the datagrams ready to read, so to the only way to know how many datagrams are waiting to be read is to call ReceiveFrom repeatedly until all data is read, for instance:-

while(Socket.Available > 0)
{
   int datagramSize = Socket.ReceiveFrom(buffer, ref endPoint);
} 

The datagramSize variable will give the size of the datagram that was read, this can be troublesome to manage since you do not know what you are going to get, until you get it, so you would need to initialize a large enough buffer to hold the datagram.

In the networking framework I am currently writing, the application has a configurable MaxPacketSize option so I can initialize my buffer to this size, but this wont help in the event that a bum packet is sent that breaches this constraint so some error handling would also need to be in place to compensate for this issue.