Android: Your mobile phone makes a lot of connections to lots of different servers. Network Connections allows you to keep a detailed eye on exactly who your phone is talking to.
The app can run in the background, keeping a live log of what your phone is doing (although more than 15 minutes requires the paid version), or it can show you just the IP addresses your phone is talking to right now. Tapping on an individual connection will give you more info on where that app is and what it's doing.
It's not a defence in itself, but it can help you find out if an app is up to no good.