Praise be to Allah.
We can tell if a hadeeth is fabricated firstly by looking as its isnaad (chain of narrators) and text. If there is someone in the isnaad who is a liar, then it is a fabricated hadeeth. With regard to the text, this can also indicate that a hadeeth is fabricated but in most cases the person studying the hadeeth needs to have experience and knowledge of the saheeh (sound) hadeeths of the Prophet (peace and blessings of Allaah be upon him).
Imam Ibn al-Qayyim (may Allaah have mercy on him) said:
I was asked: is it possible to recognize a fabricated hadeeth by following guidelines without looking at its isnaad?
This is a very serious question. That may be known to one who has deep knowledge of the saheeh Sunnahs, for whom it has become second nature and who is extremely specialized in the knowledge of the Sunnah, and reports, and has knowledge of the Seerah (biography) of the Messenger of Allaah (peace and blessings of Allaah be upon him) and his teachings, commands and prohibitions, what he told of, what he called people to, what he liked and what he disliked, and what he prescribed for his ummah, so that it is as if he mixed with the Messenger of Allaah (peace and blessings of Allaah be upon him) like one of his companions. Such a person will have knowledge of the life, teachings and words of the Messenger of Allaah (peace and blessings of Allaah be upon him), and what he could have said and could not have said, in a way that others will not know. This is the way of every follower and the one whom he follows, because the one who becomes close to the Prophet (peace and blessings of Allaah be upon him) and is very keen to find out about what he said and did, becomes more able to distinguish what may correctly be attributed to him from what is not correct, in a way that those who do not have such knowledge cannot do. This is the way of followers with their imams: they know their words, texts and points of view.
And Allaah knows best.
Al-Manaar al-Muneef, p. 43, 44.
Following these words, and until the end of the book mentioned, Imam Ibn al-Qayyim (may Allaah have mercy on him) gives many examples of fabricated ahaadeeth on various topics, with numerous comments. Reference may be made to this book as it is very valuable.
See also the answer to question no. 6981.