Check if online

You can use the Remote Access Service (RAS) function RasEnumConnections to check if there are any Dial-Up Networking connections.

Can you please specify how to use the RasEnumConnections command?
I can't make it work...

// return value of RAS functions
DWORD dwRet;

// get the number of connections
DWORD dwSizeBuf = 0, dwNumConnections = 0;
::RasEnumConnections(NULL, &dwSizeBuf, &dwNumConnections);

// enumerate the connections
if (dwNumConnections != 0)
{            
    // allocate the buffer
    LPBYTE lpbyBuf = new BYTE[dwSizeBuf];

    // enumerate the connections
    LPRASCONN lpConn = reinterpret_cast<LPRASCONN>(lpbyBuf);
    lpConn->dwSize = sizeof(RASCONN);
    if ((dwRet = ::RasEnumConnections(lpConn, &dwSizeBuf, &dwNumConnections)) != 0)
        // DISPLAYERROR(dwRet);
    else
        for (DWORD dwi = 0; dwi < dwNumConnections; dwi++)
        {
            // each connection handle is lpConn[dwi].hrasconn
        }

    // free the buffer
    delete []lpbyBuf;
}

Well, it still doesn't seems to work. When I compile my program I get a Linking error. It looks like this:
TestDlg.obj : error LNK2001: unresolved external symbol _RasGetConnectStatusA@8.
What am I doing wrong?

Yes, link with rasapi32.lib. (menu Project/Settings/Link)

Internet Shortcuts API in ActiveX SDK has a function.

InetIsOffline


BOOL InetIsOffline (
    DWORD dwFlags
);

Determines whether the system is connected to the Internet.

Returns TRUE if the local system in not currently connected to the Internet. Returns FALSE if the local system is connected to the Internet or if no attempt has yet been made to connect to the Internet.

dwFlags
Input flags for the function. This must be set to zero.

Greate! My online detector function is working just fine now. But I'm courious about the InetIsOffline() function. When I add that function to the code I am getting the same error as before, so I presume that I need to add a Library file. But which? The error looks like this:
TestDlg.obj : error LNK2001: unresolved external symbol "int __cdecl InetIsOffline(unsigned long)" (?InetIsOffline@@YAHK@Z)