A famous physician of the second century AD wrote a book explaining how to tell his real writings from the ones forged in his name. Who?

Galen. The Roman physician wrote a book explaining how his genuine writings could be told from those forged by others. Writing in a famous name was common enough that an author could need such a list. The New Testament knows the practice too: 2 Thessalonians warns of a letter "as though from us" (2 Thessalonians 2:2).