Contrary to popular belief, it's very difficult to successfully challenge a will. The two most common legal grounds for challenges are that the testator lacked testamentary capacity (or, mental competence) to make a valid will, and that the testator fell victim to the undue influence of another, so that what was written in the will doesn't reflect the testator's actual freely-made choices. The law, however, demands a strong presumption against both of these arguments.
With regard to testamentary capacity, the law doesn't really require very much in the way of mental acuity. The testator (the will's maker) doesn't have to be aware of current events, for example, or even know the date. Capacity to make a will basically only requires that the testator understand the nature and extent of his (or her) own assets, know who his family members are and that he is (or isn't) naming them in his will, and realize that in signing the will he's directing the distribution of those assets upon his death.
If the testator has awareness of these elements, then testamentary capacity exists as far as the law is concerned. As such, a lack of mental capacity at the time that the will was signed would be an extremely difficult matter to prove. And given the fact that the law requires the judge to lean toward a presumption of adequate testamentary capacity, proving otherwise would be even less likely.
Undue influence, on the other hand, is just a bit easier to prove, especially if there's evidence that the testator was mentally weak - even if not to the degree that testamentary capacity was lost. To better understand the concept of undue influence, it's helpful to delineate each component of the term. "Influence" exists when someone that has a close relationship with the testator persuades him to bequeath his property in such a manner that he would not have without the persuasion. The influence will only be considered "undue" when it increases to the level of mental coercion - in other words, when the circumstances have been taken advantage of.
Every case is judged on its own conditions and merits, therefore making it virtually impossible to depict what generally constitutes undue influence. To determine if it in fact existed at the time of the making of a will, the court will attempt to ascertain whether the testator's final decisions were freely and independently arrived at - in other words, if they were they his own. Again, the law fundamentally presumes that they were, so proving otherwise is an uphill battle.
In evaluating the competing arguments of a will contest, the court would ask questions and look for some of the conditions that experience has revealed in past cases of undue influence by a person named as a beneficiary. Some fundamental questions might include: Was the testator physically weak or mentally impaired? Was there a close, confidential or caretaking relationship between the testator and the person accused of the undue influence? If there was, was this relationship developed only recently before the writing of the will that's being challenged? The court uses no particular formula or blueprint for arriving at its' decision; it's based solely on the totality of the circumstances of each unique case.
It must be remembered, however, that almost everyone who's ever been left out of a will feels slighted and is sure to think that the testator must have been incompetent or unduly influenced by someone else. Although such a reaction is understandable (especially in situations that really do appear on the surface to be unfair), having a will invalidated on that basis alone presents a very difficult challenge indeed.