Part of the problem is about how much you're willing to upset your users. There are plenty of discussions about that, and how to find a compromise (for example show your own ads if they are offline). See discussions like this one:
http://forums.makingmoneywithandroid.com/marketing-methods/7629-how-show-ads-when-user-offline.html
The first part of the problem is knowing if you're really offline. There are ANEs like this one that can help:
GitHub - freshplanet/ANE-Network-Info: Air Native Extension (iOS and Android) for getting Network Information
Or you could simply have your own text file on your server, and your app tries to read that. If it succeeds you let them play. You could check that file every time an ad is due to play, so that someone who goes offline after starting to play will also see the message that they need to be online to play. If you do tie the check to when ads should appear, you should be fairly sure that you won't cause them to lose a life.
If your own server URL was to a PHP you could even know how many times users have played the app each day.
Hate reviews would reduce your ad revenue more than the amount that people going offline would.