Secret Chats
Official telegram.org Source
Unfortunately, there is no bulletproof way of detecting screenshots on certain systems (most notably, some Android and Windows Phone devices). 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.
Official Telegram Reference: https://telegram.org/faq
Part of the TGWAY.com Official Telegram Knowledge Base (98 Verified Q&As).