If your application must have network access the easiest way is to add the following settings to your info.plist as boolean values.
SBUsesNetwork – Ensure the device has an active connection(Edit: not applicable, this seems to be a private API someone found at some point. It is not in Apple’s developer documentation.)- UIRequiresPersistentWiFi – Ensures the device is connected via WiFi
If your choice is not true then the user will be presented with an appropriate message when starting your application. Best of all this message is from the OS and thus is localized.
If your application cannot download data from a website while running (loss of signal, site down) you should still warn the user though and not just spin indefinitely.