On newer versions of Android, you can't take screenshots in Secret Chats. This will result in the error
'Can't take screenshot due to security policy'. On older versions of Android, Telegram will display screenshot alerts whenever one is taken.
Note: We will make every effort to alert you about screenshots taken in your Secret Chats, but it may still be possible to bypass such notifications and take screenshots silently. We advise to share sensitive information only with people you trust. After all, nobody can stop a person from taking a picture of their screen with a different device or an old school camera.
Any Android device with
SDK 23 (Android Marshmallow – 6).