Often if there is evidence of undue influence then there will not be a need to prove it because lack of knowledge and approval will be easier to establish.
In probate cases, where you are challenging a Will, you have to show that there has been actual undue influence. Unlike in a lot of other legal areas undue influence will not be presumed. The burden of proving undue influence is on the person alleging it. Often that is too difficult a burden to overcome. It is not enough to show that the facts are consistent with undue influence having been applied it must be shown that the facts are inconsistent with any other hypothesis.
Persuasion, appeals to affection, gratitude for past services, pity for future suffering can all be pressed on a testator without it being regarding as undue influence. Threats, on the other hand step over the line. A testator can be led but not driven, his Will must be an expression of his wishes, not someone else’s.
Accordingly, because of the difficulties set out and the easier remedy of lack of knowledge and approval it is very unusual to pursue a challenge to a Will based solely on undue influence.