Mohd. Mohideen v/s Muthukumara Thevar 

Madras High court.

But this will not help you in case where the attesting witness has turn hostile. 

Can be done by examining witnesses who were able to prove the handwriting of the testator or executant. The burden of proof then may be shifted to others.

Sir section 69 is law itself in case the witness is not available the signature in his hand writing can be checked along with the signature of the testator.

The Court, while granting probate of the will, must take into consideration all relevant factors. It must be found that the will was product of a free will. The testator must have full knowledge and understanding as regards the contents thereof. For the said purpose, the background facts may also be taken note of. Where, however, a plea of undue influence was taken, the onus wherefor would be on the objector and not on the offender. {See Savithri & Ors. v. Karthyayani Amma & Ors. [JT (2007) 12 SC 248]} 

https://www.advocatekhoj.com/library/judgments/index.php?go=2008/april/207.php

In case of hostile there has to be other majors taken.

the attesting witnesses are really dead the plaintiffs can very well prove the will by resorting to Section 69 of the Indian Evidence Act by examining one of the persons who has acquaintance with the signature of any one of the attesting witnesses and also the executant of the document. Before resorting to the provision under Section 69 of the Indian Evidence Act to prove the will, it must be established by the plaintiffs the factum of death of attesting witnesses. Mere pleading in the plaint as to their death itself is not sufficient for establishing the factum of the death of the attesting witnesses. The plaintiff ought to have filed death certificates of the concerned attesting witnesses to prove their alleged death.

Madras High Court

Raniammal vs A.Saroja

It can be proved by proving the signature of the testator it's difficult but not impossible. 

the section itself is clear

why you need judgments for the same

if the attesting witnesses are dead, then the propounder of the Will can rely on the evidence of any other person who has knowledge about the testator signing the Will

if the witnesses turn hostile then the propounder has to simply apply to the court for issuing witness summons to such witnesses

In the event that no such attesting witness is alive or can be found, then as per section 69 of the Indian Evidence Act, 1872, the Will has to be proved by proving the signature of the testator as well as that of at least one attesting witnesses.