Android: Your desktop's webcam probably has a handy indicator to tell you when it's on. Your phone does not. If you're concerned about certain apps using your camera or microphone when you're not aware of it, D-Vasive attempts to help keep you aware.
The app will put a notification in your shade whenever one of the sensors you're monitoring becomes active. You can then tap that notification and dive into the log files to find out which app is the culprit. The free version only allows you to monitor your sensors, but the paid version ($5) allows you to block access as well.