who can prove the "will"

The witnesses, who has signed the Will.

If any person shall attest the execution of any will or codicil to whom any beneficial devise, legacy, estate, interest, gift, or appointment, or affecting any real or personal estate, other than and except charges and direction for the payment of any debt or debts, shall be thereby given or made, the devise, legacy, estate, interest, gift or appointment shall, so far only as concerns that person attesting the execution of the will, or codicil, or any person claiming under that person, be utterly null and void; but the person so attesting shall be admitted as a witness to prove the execution of the will, or codicil, or to prove the validity or invalidity thereof, notwithstanding the devise, legacy, estate, interest, gift, or appointment, mentioned in the will, or codicil. Also See Section 63 of the Indian succession Act, 1925  and Section 68 of the Indian Evidence Act. 

Burden lies on both who challenges and in whose favour will is executed. The person in whose favour will is exeucted he has to prove the will thru., the wittnesses, scribe of the will is most important and one of the wittnessess who signed to the will.

It will proved in court through probet.