You need to try to connect, there is no way find on which port someone is listening without directly asking, hey, is there anyone on this port?

Here is the way to do so from C#:

TcpClient tc = null;
try
{
    tc = new TcpClient("MyAddress", MyPort);
    // port is open
} 
catch(SocketException se) 
{
    // port is not open, or host is not reachable
}
finally
{
   tc.Close();
}

Well you must know on which port the Sql server services are listening on remote server. Default port is 1433. If you want to test if a port is open on a server or not then you can try Telnet ping.

open command prompt:

C:\> Telnet <YourServername> <portno.>

If it's open then a screen will go empty immediately or return an error.

You need to know the IP address or host name of the server and the TCP port, too. The instance name alone won't help, as they are not advertised on the network. If you have the server's name or IP and the SQL Server instance name, you can simply try to connect using an SqlConnection with an appropriate connection string. If you can connect, everything's fine.