1) understand the nature of the testamentary act;
2) understand and recall the nature and situation of their property;
3) possess knowledge of the people who are the natural objects of their bounty; and
4) understand the manner in which the disposition of their property is to occur.
So, it may not be as simple as asking the person signing the will if they know today's date or their name. It's a little more complicated than that. This issue is one of several reasons why litigation during probate is such a point of contention. If a family member who is either going to receive very little or nothing from someone's will, they may attempt to argue that the will is invalid because the testator did not have sufficient capacity to sign the will in the first place. The effect is that the court could determine that the will is invalid, which would then make it as if the person died without having a will. The laws of intestacy would then determine the distribution of the decedent's assets. Still the court would have to rule on the issue of mental capacity, which is not an easy task.
This is why it's extremely important to have estate planning documents executed well before lack of mental capacity is ever an issue. On many occasions I had family members call me asking to make a Last Will and Testament for their loved ones, only to determine that they are in the late stages of dementia. At this point, it's too late to sign any estate planning documents. The next step would be to petition the court to be appointed a Guardian in order to sign legal documents on their behalf, except for a Last Will and Testament, which can only be signed by the testator. Expect the Guardianship process to be costly and time consuming. So, don't wait to execute your docs, as it's best to spend a little bit of money now, then to spend a lot more later.
Learn more about the basics of Last Will and Testaments and the different types of Florida Wills.