What kinds of tests can one reasonably apply to wills to check to see that each stipulation holds up without going before a judge and can withstand a courtroom challenge?
The requirements for a valid will vary from jurisdiction to jurisdiction. In most U.S. states there is a two part test. One part is determined based upon the face of the document, the other is based upon extrinsic evidence.
To be formally valid without a hearing unless an interested party requests one in most U.S. states on its face (i.e. to be a "self-proving will") a will must:
Be in writing.
Be signed by the person making the will who is at least eighteen years of age.
Be signed by at least two witnesses who do not have conflicts of interest, who are attesting that the person making the will was competent and that they saw him or her sign it.
The signatures of the witnesses must be notarized. If their statements are not notarized, usually at least one witness must testify in a hearing or with a later affidavit.
The language of the will must indicate an intent to make a binding disposition of property in the estate of the person making it when they die, and other rights related to their estate and their death.
Various exceptions to these requirements exist in most state (e.g. a "holographic will" is valid if it is in writing, indicates the necessary intent, is signed, and has its material provision substantially in the hand writing of the person writing it).
Some states allow oral wills called "nuncupative wills" in very specific circumstances usually limited to soldiers or sailors in circumstances of actual battle or imminent mayhem likely to result in death, with the proper witnesses.
A will is like a dollar bill, a copy is not as good as the original document. If the original document cannot be found it is presumed that the will was revoked by physical destruction of the will, in the absence of evidence to the contrary submitted at a hearing to a judge.
Any interested party can contest a will that appears valid on its face under the standards describe above on several grounds.
Two of these relate to the condition of the person signing the will at the time it was signed. The notarized witness statements create a presumption that these two conditions were met. But, that presumption can be overcome with other evidence from witnesses and/or documents or other kinds of evidence. These conditions are:
The document was procured from the person making it with duress or undue influence, even if the person making it did have "testamentary capacity."
The person making it lacked testamentary capacity which is also a two part test. The first part is that the person making it knows who their family and the "natural objects of their bounty are", knows in general terms what they own, and knows what a will is intended to do and is capable of understanding the general outlines of what it does. This basically rules out people with severe dementia. The second part is that if the person is not suffering from dementia but is suffering from hallucinations, that the hallucinations have not influenced the nature of the provisions contained in the will.
In practice, "undue influence" is the predominant ground for contesting wills that are formally valid on their face because testamentary capacity is a very low standard.
A will can also be contested on the grounds involving extrinsic evidence that:
A later validly executed document revoked the will, or
The document is a forgery.
If any of the issues related to extrinsic evidence are disputed, both the party contesting the will, and the party defending it, has a right to have that issue resolved in a jury trial, although often the right to a jury trial in a case like this is waived by the parties.
This is determined by making a reasonable inquiry before submitting the will and examining the physical document and the place where it was when it was discovered, closely. The inquiry should be very searching if the terms of the will are surprising, and can be fairly lax if the terms of the will are what was widely expected.
There is generally a notice given to all interested parties when a will is submitted to a court with probate jurisdiction to be given effect that contains a deadline to raise a contest along these lines. A will that is not submitted to a court with probate jurisdiction usually has no force or effect until that happen in most U.S. jurisdictions.
Even if the will is valid in general, some terms of the will may be invalid as a matter of public policy, or modified by statue, in certain circumstances. These exceptions can usually be determined on the face of the document or with minimal additional information (although "your mileage may vary" and cases where, for example, the cause of death or date of death is uncertain can be difficult).
For example, the following limitations are common:
A term in a will conditioning a gift at death on marrying or not marrying a particular person is void in most, if not all, U.S. states.
Under certain circumstances a will is required to leave certain minimum amounts to a surviving spouse and/or minor dependent children.
If a will appears to omit a child of the person making it by accident, it is modified in particular ways mandated by statute.
Will provisions making gifts at death or giving responsibilities to someone who murdered the person who wrote the will are void.
Some states invalidate provisions in favor of an ex-spouse upon divorce, unless a post-divorce document reaffirms those inheritances.
Certain kinds of words are given defined statutory meanings unless the express language of a will says otherwise, such as "by representation", or "if he survives me", that are not necessarily intuitive or in line with every day use of language.
It is usually necessary to review the probate court and case law in the state where the decedent was domiciled at death to determine if these limitations apply, ideally with a probate or estate administration lawyer.