about-device
Go to Settings>About Device or About Phone

Android

  • Android 4.4 to Android 6.0.1 (Android Kitkat and above)
    (Android N will not be supported until the official Android release)
  • Preferred resolution of 720×1280 pixels (Not optimized for tablet)
  • Strong internet connection (Wi-Fi, or 3G and  4G mobile DATA)
  • GPS and Location Services

iOS

  • iPhone 5+
  • iOS 8+
  • Strong internet connection (Wi-Fi, 3G, or 4G)
  • GPS and Location Services
  • Jailbroken devices are not supported

Source: https://support.pokemongo.nianticlabs.com/hc/en-us/articles/221958248-Supported-devices

To check the version of your current Android OS, go to settings>about device. If your device is not compatible with Pokemon Go or you can’t see the game in Play Store or App Store, you might want to try running a software update because you can’t really install the game if your device is Android Jellybean 4.2.1 and below or iOS7 below.