There isn’t an indicator on your smartphone that lets you find out if your camera or mic is on. Now, if this is an issue for you, D-Vasive is designed to allay your fears. The app notifies you if one of the sensors you are monitoring becomes active. It will let you look into the log files to see which app is doing so. The free version of the app will only notify you of the active sensors but if you are willing to shell out US$5, you can block access as well.
Source: Lifehacker | Download: Google Play Store (Free; US$5)