Google updated its security features by introducing Verify Apps feature. It helps you to install applications from sources other Google Play more confidently. This features is automatically activated and when your are about to install an applications, you can choose to verify app before installation. If this features is disabled on your Android device, follow these steps to enable it:
1. Go to App Drawer and open Google Settings
2. Tap Verify Apps
3. Make sure "Verify Apps" is checked, so Google can scan apps to make sure no virus or harmful Malware is threatening your device.