What are the techniques for distinguishing sahih and fake Hadiths?
What should we do when we hear a new Hadith? Should we immediately accept it as sahih until someone proves that it was fake, or should we be doubtful from the beginning?
Is there any database or Hadith collection/book where we can check if a random Hadith is sahih or fake?
Checking authenticity of hadith needs expertise and it is not a simple matter that one can do on himself/herself so a non-expert should be very careful in stating and using hadith. There is an Islamic science that studies hadith and their authenticity and it is dependent on several other Islamic sciences (e.g. "علم الرجال").
Hadithes are historical documents. The way to check the authenticity of a hadith is similar to the way of checking for the authenticity of other historical documents. A major difference is that many hadith has been preserved orally and not in a written form at least for the early centuries so there is no written document dating back to the original time the hadith has been stated. But still some means of checking authenticity of documents still apply, for example existence of multiple independent lines of narration from the source increases the confidence in the authenticity. Usually the persons who have narrated the hadith are also important for confidence.
Another basic rule for checking authenticity is that hadith should not contradict what is said in Quran so one also needs to be knowledgeable in Quran and its interpretation which is again a non-trivial Islamic science.
There are several major collections of hadith. Some of these are solely collection of all hadith that the author was aware of without vetting for authenticity (the main reason making all claimed hadith available for experts without bias). Others are collections where various amounts of vetting is performed. There are some hadith collection which are considered more reliable (though even in those books you may find contradictory hadith).
Shia and Sunni often have different collections because they consider different people in Islam to be reliable sources of hadith, e.g. Shia do not consider a person a reliable source simply because he or she was a companion of the prophet. In cases where there are contradictory hadith (and historically we know there were because of internal wars between Muslims not long after the prophet has passed) one needs to either choose a source to rely on and reject the other or possibly accept only those hadith which are agreed on by all sides.
The Sunni consider the following collections as reliable:
On the other hand Shia consider the following collections as reliable:
Even if you take a single collection from the 9 collection mentioned above you may find contradictory hadith. I personally suggest you to be very careful about narrating a hadith unless you have studied the required sciences to be able to distinguish between authentic and non-authentic ones and when in doubt do not accept a hadith following Quran's advice (c.f. verses 17:36, 29:8, 31:15), particularly when you see it is not consistent with Quran and hopefully God will guide us to his path. (We are quite sure that Quran is very authentic, although we know there has been minor differences as we have around 14 recitations of Quran, the differences between these narrations are small and reasonable considering the fact that Quran was not collected as a written book and was preserved orally during the prophet's time.)
A major difference is that many hadith has been preserved orally and not in a written form at least for the early centuries so there is no written document dating back to the original time the hadith has been stated. Not true. There have been entire disserations and treatises written about the fact that there was written hadith/compilations in the first century AH. See the Sahifah of Hammam ibn Munabbih: islamic-awareness.org/Hadith/hadith.html
The sciences which deal with checking a Hadith's authenticity fall under Uloom Al Hadith (The sciences of Hadith). An example of one of the sciences which deal with checking the authenticity of a Hadith is Al Jarhu Wa Atta'deel. Ajjarhu Watta'deel (or Ilm Rijalul Hadeeth), goes through the sanad/chain of transmission and sees the states of the narrator, whether they are trustworthy/righteous or not, whether they are accurate (whether in memory or written form or both) or not and so on.
Scholars of Hadith of the past have gone through many Ahadeeth and checked them. A reliable modern scholar of Hadith, known as Nasiruddeen Al Albany has done a massive work by going through the Sunan and checking the authenticity of the Ahadeeth. The Authenticity of a Hadith should be known before using it, hence to find the authenticity of a Hadith one must do research on it, and see what Scholars of Hadith have said in regards to it.
It should also be noted that checking a Hadith is not simple task, and needs much knowledge and expertise.
Main Sources: Islamic online university, and Mandhoomat fi Qawa'id Ajarhu Wa Atta'deel (منظومة في قواعد الجرح و التعديل)
@AlUmmat's answer is correct and is a technical answer. The science of hadith (which grew primarily around 200 AH, or 800 CE) dealt with distinguishing weak vs. authentic vs. fake.
For our times today, all of the hadith have been collected into major books. Knowing which hadith in which book, along with which label (weak/daeef, good/hasan, strong/saheeh, or fabricated/mawdoo) is sufficient to tell you what the hadith is.
In general, when a website, lecture, class, etc. quotes a hadith, they will tell you if it is weak/strong/fabricated. If not, you can ask. Islam puts a lot of emphasis on not attributing anything to Prophet Muhammad (peace be upon him) that he didn't say; we're very stringent about making sure we know if the hadith is authentic or not.
IMO there is a fundamental flaw in the question. The word "Sahih" as has been used by the Muhaddisin doesn't mean or imply authenticity. It is a technical term and has a different connotation when used in academia. Its wrong to assume that the opposite of "Sahih" is "Fake". The word "Sahih" doesn't convey if a Hadees is authentic or true.
Sahih is a classificatory term just like an other terms used for Hadees. E.g.,
A hadees gets classified as "Sahih", or "Mutatwatir", or "Mouzu", if it meets the classification criteria defined for that class by a Muhaddis. A "Sahih" classification DOESN'T imply that a hadees is divinely inspired or that it is sacred. It is NOT any of that. Its unfortunate that this technical term has continually been used for painting things as black and white.
if quran is a word,then hadish is the meaning,now to understand the quran should be in the same way without distorting the meaning as momin understood,surah alnisa 115,to have iman on nabi and to follow nabi,at least one book is needed which is complete in all respect,that is it has iman,islam,sunnan/ahkam,sharia,haram o halal etc,shahih bukhari was collected after 190 hizri by imam bukhari r.a has all subjects as described above,it is not so shahih/correct book as imam said,because all the mohaddesin have cheked its verification for authenticity,all the mohaddesin have the memory and actions of these hadishes much before these collections,shaih bukhari is unique in the collections of isnad of all motassil/connected rawi/speakers and the matans/subject matters free from all the defects which is described the true tunes of language of nabi,it is to be noted that different persons have different tunes of the same language,now these books are openly placed to all the scholars to check/verify its isnad till judjement day
for newcomers,a thumb rule to accept the hadish,there are four categories of hadish books,a) shahih bukhari and shaih muslim b)abudawood,trimidi,nasai,ibn e maja c)mowta imam malik and musnad ahmed d)ibn e khazima,ibn e ahban,mustard hakim."a" contain 7500 hadish with many common is called sahiheen/shahih hadish,"b" contains the mixture of shaih and weak,the writers/imam of books mentioned the qualities of hadish,"c" and "d" also contains strong/weak hadish,and the mohaddesin have a done a lot of jobs signifying the qualities of hadish,the hadish does not contain in thes holy books of hadish ,much in chance to fall in the category of mozoo/batil rewayat,to which nabi s.a.w had already warned these will be happened,it is to be noted that six books of (a) and (b) are said to be the saha satta/six true books,if a hadish is found in bukhari and muslim both,then it is 100% true hadish to be followed.
