What networking/system calls can be used to determine if the machine my software is running on is connected to the network? We are using sockets to do HTTP comms but have discovered no way to get the network status, for instance, when someone disconnects the network cable or the network goes down. Any suggestions/ideas/recommendations will be greatly appreciated!

This is heavily dependent on the OS you're running. In Unix based systems you can issue various ioctls to various network devices to see their status (not sure on all the exact details). There are a couple of quick and dirty ways to try to do this though, such as using popen to run ifconfig and then parsing the output or trying to ping a remote host.

Usually with nextwork programs, you just try to do a bind/accept or connect (depending on whether you're a server or a client) with your socket, and if it doesn't work report the error to the user.