The nature and standard of evidence required to prove a Will:
1. Stated generally, a Will has to be proved like any other document, the test to be applied being the usual test of the satisfaction of the prudent mind in such matters. As in the case of proof of other documents, so in the case of proof of Wills, one cannot insist on proof with mathematical certainty.
2. Since section 63 of the Succession Act required a Will to be attested, it cannot be used as evidence until, as required by section 68 of the Evidence Act, one attesting witness at least has been called for the purpose of proving its execution, if there be an attesting witness alive, and subject to the process of the court and capable of giving evidence.
3. Unlike other documents, the Will speaks from the death of the testator and therefore the maker of the Will is never available for deposing as to the circumstances in which the Will came to be executed. This aspect introduces an element of solemnity, in the decision of the question whether the document propounded is proved to be the last Will and testament of the testator. Normally, the onus which lies on the propounder can be taken to be discharged on proof of the essential facts which go into the making of the Will.
4. Cases in which the execution of the Will is surrounded by suspicious circumstances stand on a different footing. A shaky signature, a feeble mind, an unfair and unjust disposition of property, the propounder himself taking a leading part in the making of the Will under which he receives a substantial benefit and such other circumstances raise suspicion about the execution of the Will. That suspicion cannot be removed by the mere assertion of the propounder that the Will bears the signature of the testator or that the testator was in a sound and disposing state of mind and memory at the time when the Will was made, or that those like the wife and children of the testator who would normally receive their due share in his estate were disinherited because the testator might have had his own reasons for excluding them. The presence of suspicious circumstances makes the initial onus heavier and therefore, in cases where the circumstances attendant upon the execution of the Will excite the suspicion of the Court, the propounder must remove all legitimate suspicions before the document can be accepted as the last Will of the testator.
5. It is in connection with Wills, the execution of which is surrounded by suspicious circumstances that the test of satisfaction of the judicial conscience has been evolved. That test emphasises that in determining the question as to whether an instrument produced before the Court is the last Will of the testator, the Court is called upon to decide a solemn question and by reason of suspicious circumstances the Court has to be satisfied fully that the Will has been validly executed by the testator.
6. If a caveator alleges fraud, undue influence, coercion, etc. in regard to the execution of the Will, such pleas have to be proved by him, but even in the absence of such pleas, the very circumstances surrounding the execution of the Will may raise a doubt as to whether the testator was acting of his own free Will. And then it is a part of the initial onus of the propounder to remove all reasonable doubts in the matter.
Source: M A Rashid, Supreme Court Guidelines and Precedents