If you're creating a network application, often you need to know if you have an active network connection to a LAN.  (Note that this is not the same thing as determining if you have a valid Internet connection.)  Winsock has a function which will let you get a list of available network interfaces:  WSAIoctl.

Using WSAIoctl, you can get a list of interfaces on the computer as well as some status information about them.  For example, it will tell you if the connection is up, loopback, point-to-point, supports multicasting, or supports broadcasting.

To get the list, you would use:

INTERFACE_INFO interfaces[32];
unsigned long nReturned = 0;
int nRet = WSAIoctl(s, // socket handle
    SIO_GET_INTERFACE_LIST, 
    0, 
    0, 
    &interfaces,
    sizeof(INTERFACE_INFO) * 32, 
    &nReturned, 
    0, 
    0);

if (nRet == SOCKET_ERROR) 
{
    int nError = WSAGetLastError();
    _tprintf(_T("Error getting interface list: %i\n"), nError);
    return;
}

After this, your list of interfaces will be in the array and the number of interfaces can be determined by:

int nNumInterfaces = nReturned / sizeof(INTERFACE_INFO);

From here, you can iterate through the interfaces checking for a connected interface.

for (int i = 0; i < nNumInterfaces; ++i) 
{
    INTERFACE_INFO *pIf = &interfaces[i];
    if ((pIf->iiFlags & IFF_LOOPBACK) != 0)
        continue;

    if ((pIf->iiFlags & IFF_UP) == 0)
        continue;

    // If you get here, you have a valid
    // interface to the LAN.
}

In the above case, we're looking for an interface that is up and not a loopback.

There is some other useful information in the INTERFACE_INFO structure:  ip address of the interface, netmask, broadcast mask, whether the connection is point-to-point, and supports multicasting or broadcasting.

I will reiterate that this does not tell you if you are connected to the Internet, nor will it tell you whether there's anything on the other end to communicate with.