Test your app with Airplane Mode enabled!
Depending on your priorities, this can mean getting the app to work almost perfectly in offline mode — i.e. caching enough data so that you can fulfill most of user needs (e.g. maps apps or document viewer apps) and persisting user operations that require connectivity (such as posting a tweet) so they can be "played back" when there's connectivity — to something similar like displaying an alert to the user notifying that the app needs connectivity to work.
At the very least, you should check that the app doesn't break without connectivity, e.g. overriding your app's local cache with empty or "nil" data because of failed network requests.