These are the bare basics needed for your network to get on the Internet, in the order they are used:
Network Interface (NIC) -> IP address -> Default Route -> Gateway(public) IP address -> DNS server -> WWW
Here is a command line bash script that will perform the test for you and let you know what problems exist, if any. The only thing you as a user need to do is set the Public IP address variable (PUBIP) in the script, for your individual network.
Put it in /my-applications/bin, then just run the 'chklan' command from any console
[edit] I changed the PUBIP variable to "0.0.0.0" so it doesn't fail initially. User needs to set it to their public IP when possible.
This utility should work with any Linux that uses the bash shell.