This is the one that gained the most traction throughout the first season. The biggest clue is that some people call the devil "Old Nick" and some call him "Mr. Scratch." Nick's interest in Sabrina is almost as obsessed as the Dark Lord's — like, beyond the level you'd expect from a typical boy fascinated by the cute new girl in school. You can never quite tell if his nice guy act is sincere. Reddit really likes this one, and they have many more examples of possible hints here