Watch your devices' activity lights. While playing by yourself your network activity should be pretty much nil, however when someone else joins to hack/trail you, the game has to stream data across the network. So if you notice the activity lights on your router/modem/switch* start flickering constantly all of a sudden, there's a really good chance someone is in your game. * Depends on how your network is configured. If connecting wirelessly, you'll need to watch your access point's lights (which is probably also your router). If you're wired, watch the lights of the device closest (along the network path) to whatever you're using to play it on.