The burden of proof in a will contest varies based on the time that the will contest is filed. Generally, if a will contest is filed before the Judge admits the will to probate, the person who filed the will for probate (the proponent) will have the burden of proof. If the will contest is filed after the will is admitted to probate by the Judge, the person opposing the will (the opponent) will have the burden of proof.
It is helpful to have your adversary shoulder the burden of proof. Therefore, if you are going to contest a will, you should contact an attorney as soon as possible. While having or not having the burden of proof in a will contest case is not determinative of the case, it is helpful for the other side to have that burden.