if someone is blaming that ,your near one had sex abuse with their child ,and the person refuse for not doing it,how you will come to know who is right?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

It is Islamically established that in principle a Muslim is innocent, so it is not permissible for someone to accuse him of something abominable without evidence; Allaah Says (what means): {O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin.}[Quran 49:12]

Based on the innocence of the accused, he is the one who should be believed unless the contrary is proven; for more benefit, please refer to Fatwa 138768.

Finally, it should be noted that a Muslim should fear Allaah in regard to the honor of his fellow Muslim; Abu Hurayrah narrated that the Prophet said: “All things of a Muslim are inviolable for another Muslim: his blood, his wealth and his honor.”

Allaah Knows best.