Unfortunately, there's no way to get a completely effective tool to advise about a screenshot. There are different ways to bypass this feature (as taking a picture with another device or using external apps).

By now, the way to get those screenshots is recognizing them with the word 'screenshot' in the path of the picture. So, once it's done, the notification is sent to the other user.

We already know about two manufacturers that have tools to avoid it: LG and Samsung.

Other ways are to change the code and disable this element, or saving the picture directly from the cache. So, as you see, there are options to get that picture.