When it's made of skin, has words such as "DO NOT READ" and "DEVIL'S B*TCH" written on it. As well as stuff like the satanic star, things that says "IT WATCHES". Also when the book is wraped in barbed wire. That sorta thing.
Oh! And when it is surrounded by dead cats hanging from the ceiling
Sounds like I would need to call an Imperial Librarian, they would know how to properly dispose of the heretical literature. I only read Imperial Consortium approved literature.