Praise be to Allah.
Determining whether a hadeeth is saheeh or da‘eef comes after a great deal of study, that passes through two main stages:
researching all the places where the hadeeth appears in all of the books of the Sunnah, to the best of one’s ability; compiling the isnaads (chains of narrators) through which it was narrated; determining the points at which these chains of narration meet and diverge; defining the chain of narration that the hadeeth is based on and noting it, in order to move on to the second stage
studying the isnaad or various isnaads of the hadeeth. This is a detailed study of all factors that may impact the ruling on the hadeeth. This is done by means of the following:
1. Researching the caliber of the narrators and the extent of their religious commitment and honesty
2. Researching the memory of the narrators (i.e., how good their memory was) in the chain and the extent of their precision in narrating hadeeth
3. Researching the connections in the chain of narrators to establish whether each narrator took it from his shaykh who narrated it to him, and that there is no interruption or tadlees [tadlees is when a narrator narrates a hadeeth that he did not hear directly from his shaykh, without mentioning the name of the third party from whom he heard it] or irsaal [irsaal refers to when a hadeeth is narrated from a Taabi‘i and attributed directly to the Prophet (blessings and peace of Allah be upon him), without mentioning the name of the Sahaabi who narrated it].
4. Researching whether the isnaad and matn (text) of the hadeeth are in accordance with other hadeeths and do not differ from them or contradict them. This is also very important.
5. Establishing that the hadeeth is free of hidden faults that no one can detect except highly competent scholars.
Once the study of the hadeeth has gone through these two stages, then it is possible to determine whether it is saheeh or da‘eef. This – as you can see – is a difficult task, that requires a great deal of time, effort and knowledge of the sciences of hadeeth and the methodology of the hadeeth scholars. It is not possible to explain it in detail in a brief answer to be published on a website. But it is a complete field of knowledge that is studied by students in colleges and universities, that they take as a specialty.
But here we should point out some guidelines that will help the Muslim who is not specialized in the study of Islamic sciences to have an idea about hadeeth classification and how to tell whether a hadeeth is saheeh or da‘eef:
1. If the hadeeth was narrated by Imam al-Bukhaari in his Saheeh with its isnaad, then it is a saheeh hadeeth.
2. If the hadeeth was narrated by Imam Muslim in his Saheeh, then it is a saheeh hadeeth.
3. If the hadeeth was narrated by Imam Maalik in his Muwatta’ with its uninterrupted isnaad, then it is a saheeh hadeeth.
4. If Imam Ahmad, Abu Haatim, Abu Zar‘ah, al-Bukhaari, Muslim, Abu Dawood, or ad-Daaraqutni ruled that a hadeeth is saheeh or da‘eef, and no imam of similar standing disagreed with him, then it is as they said.
5. If you looked for the hadeeth in the books of the scholars, or on the website ad-Durar as-Saniyyah, and you looked carefully, and you found that the scholars were agreed that the hadeeth is saheeh or da‘eef, then it is as they said, because the ummah of Muhammad (blessings and peace of Allah be upon him) cannot agree upon misguidance.
6. If you find that the text of the hadeeth speaks of some matter concerning which the scholars stated that there is no saheeh hadeeth concerning that matter, then this indicates that the hadeeth is da‘eef and not saheeh. You can find the topics concerning which there is no saheeh hadeeth in several books, such as al-Manaar al-Maneef by Ibn Qayyim al-Jawziyyah, and at-Tahdeeth bima qeela la yasuhh fihi Hadeeth by Shaykh Bakr Abu Zayd.
7. Any hadeeth which was narrated only in the following books, and was not narrated by the authors of the famous Sunans and Musnads, is a da‘eef hadeeth. These books are: ad-Du‘afa’ al-Kabeer by al-‘Aqeeli; al-Kaamil fi’d-Du‘afa’ by Ibn ‘Adiyy; Tareekh Baghdaad by al-Khateeb al-Baghdaadi; Tareekh Dimashq by Ibn ‘Asaakir; Nawaadir al-Usool by al-Hakeem at-Tirmidhi; and Musnad al-Firdaws by ad-Daylami. You can find out whether the hadeeth was narrated only by one of these authors via as-Suyooti’s book al-Jaami‘ al-Kabeer. In fact many of the hadeeths that were narrated only by Ibn Maajah, to the exclusion of the other authors of the six books, or were narrated only by at-Tabaraani, Abu Nu‘aym, Ibn Hibbaan, al-Haakim, ad-Daaraqutni, al-Bayhaqi and others like them are da‘eef hadeeths, even though (their books) also contain many saheeh hadeeths that were narrated only by them.
These are a few general guidelines that will help you to distinguish between saheeh hadeeths and da‘eef ones. We should also point out that there are some exceptions to these guidelines, but they do not affect the guidelines mentioned above.
And Allah knows best.