How to know if the will is genuine or not when the person is dead

my brother is claiming an ancestral property under a will executed by my father and written by my grandfather(property is my grandfathers self acquired).. is it necesaary that there should be my fathers signature in it..the witnesses at that time would be dead by now...its a fake will ...because till now none of my family elders know about it.. doubt over the genuinity arises because in that property through that will half share was given to my uncle and half share was given to my brother instead of my father...but my uncle had never told us about any such will..after my uncles death my brother came up with above said will and created another document as if it was sold to my another brother..please help me how to solve this A) please let me know what are the things that are infavour in the above said to proove it as a fake one. B) my doubt is if no share was given to my father why he will execute that will? * benificiary (brother) age at the time of will is approx 6 or 7 yrs