Testamentary capacity, like the capacity to contract, is usually presumed such that it would be up to someone seeking to contest a will on that basis, to prove otherwise based on a balance of probabilities.
In Banks v Goodfellow, Justice Cockburn wrote:
"In deciding upon the capacity of the testator to make his will, it is the soundness of the mind, and not the particular state of the bodily health, that is to be attended to; the latter may be in a state of extreme imbecility, and yet he may possess sufficient understanding to direct how his property shall be disposed of; his capacity may be perfect to dispose of this property by will, and yet very inadequate to the management of other business, as, for instance, to make contracts for the purchase or sale of property. For, most men, at different periods of their lives, have meditated upon the subject of the disposition of their property by will, and when called upon to have their intentions committed to writing, they find much less difficulty in declaring their intentions than they could in comprehending in business in some measure new."
However, proving a lack of the legal ability to issue a valid will is not always difficult. For example, the testator may not of been of the age of majority (i.e. a minor) when the will was signed and in a jurisdiction that sets age limits on who can make a will (eg. ยง7 of the Wills Act, RSBC 1996, Chapter 489).
Other hindrances to testamentary capacity include mental disability if such mental disability is such that the purported testator was, at the time of signing the will, unable to understand what he or she was doing; a will was or what it does. Age does not, without more, cause any presumption of mental disability.
Conversely, certain debilitating physical disabilities can extinguish, temporarily or permanently, testamentary capacity (eg. coma).
The presence of suspicious circumstances may also give rise to questions as to testamentary capacity.
Furthermore, most of the impediments to capacity known to contract law apply as well to wills and affect testamentary capacity, such as undue influence or duress.
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