Answer:
A will is legally bound if; The person in question is of sound mind when the will is drawn and signed, If the person in question has signed a waiver being of sound mind to allow another person to act on his or her behalf or if a court has made a judgment stating as much.
These are the only normal determinations for a valid will.
There are other circumstances where a will is challenged, however, this requires a strong legal case to have any of the above norms challenged.