How do we know if a Hadith is fabricated or not?

A fabricated Hadith (definition) can be identified by examining its chain of transmission (Sanad): if it contains a liar, then it is fabricated. Alternatively, it can be identified by examining its text (Matn), though this requires expertise and knowledge of sound Hadith. Ibn al-Qayyim explained that identifying a fabricated Hadith by a criterion without looking at its Sanad requires deep knowledge of sound Sunnahs, the Prophet's biography (Sirah), and his guidance, such that the observer develops an innate ability to distinguish what can be attributed to him from what cannot.