Sorry but I've played 3-4 games now and I still don't get it, how the hell do I catch spies? There must be SOME way of absolutely guaranteeing that no spies get into your cities? I mean sometimes I 'stumble upon' spies every turn and sometimes not for an entire game .. What has an impact on whether I stumble upon them or not? And is there any guaranteed way of ensuring they don't wreak havoc in your country? 
