Since the Qur’an has in no way obligated us to adopt a particular method in proving a crime, it is absolutely certain that a crime stands proven in Islamic law just as it is done so in accordance with the universally acceptable methods of legal ethics endorsed by sense and reason. Consequently, if circumstantial evidence, medical check-ups, post mortem reports, finger prints, testimony of witnesses, confession of criminals, oaths and various other methods are employed to ascertain a crime, then this would be perfectly acceptable by the Islamic law. 
     It is to this fact that the Prophet’s words ‘to substantiate a crime is the claimant’s responsibility, and the person who refutes it will have to swear an oath’ allude to. 
     In the words of Ibn Qayyam: 

The word ‘bayyinah’ in the language of the Qur’an, of the Prophet (sws) and of his Companions is the name of everything by which the truth becomes evident. Hence contrary to its connotations in the terminology of the jurists, it has a wider meaning because they only use it for two witnesses or an oath and a witness. 

     However, there are two exceptions to this: 
     Firstly, if a person accuses a chaste and righteous man or woman having a sound reputation of fornication. In this case, the Qur’an stresses that the accuser shall have to produce four eye-witnesses. Anything less than this will not prove his accusation. Circumstantial evidence or medical examination in this case are absolutely of no importance. If a person is of lewd character, such things have a very important role, but if he has a morally sound reputation, Islam wants that even if he has faltered, his crime should be concealed and he should not be disgraced in the society. Consequently, in this case, it wants four eye-witnesses to testify and if the accuser fails to produce them, it regards him as guilty of qadhaf. The Qur’an says: 

Upon those who accuse honourable women [of fornication] and bring not four witnesses as evidence [for their accusation], inflict eighty stripes, and never accept their testimony in future. They indeed are transgressors. But those who repent and mend their ways, Allah is Oft-Forgiving and Most-Merciful. (24:4-5) 

     Secondly, to purge an Islamic state from prostitutes who, in spite of being Muslims, do not give up their life of sin, the only thing required, according to the Qur’an, is that four witnesses should be called forth who are in a position to testify that a particular woman is a prostitute by profession. In this case, it is not necessary at all that they be eye-witnesses. If they testify with full responsibility that she is known as a prostitute in the society and the court is satisfied with their testimony, then they can be given any of the punishments fixed by the Qur’an for habitual criminals. The Qur’an says: 

And upon those of your women¹ who habitually commit fornication, call in four people from among yourselves² to testify over them; if they testify [to their ill-ways], confine them to their homes till death overtakes them or God finds another way for them. (4:15) 

     Whether it is the question of four witnesses in these two cases, or of other evidence in any other crime it is left to the discretion of the judge whether he accepts someone as witness or not. In this regard, there is to be no discrimination between a man or a woman³. If a woman testifies in a clear and definite manner, her testimony cannot be turned down simply on the basis that there is not another woman and a man to testify alongside her. Likewise, if a man records an ambiguous and vague statement, it cannot be accepted merely on the grounds that he is a man. If a court is satisfied by the statements of witnesses and by any circumstantial evidence, it has all the authority to pronounce a case as proven and if it is not satisfied, it has all the authority to reject it even if ten men have testified.