You can contest the validity of a will by making one or more claims that, if proved, would make the will, or provisions in it, unenforceable. Depending on the law of the particular state, the legal grounds can be fraud, duress, undue influence, lack of capacity, improper execution or forgery.
The courts consider the fact that the testator can no longer testify, and put the burden of proof on the person that brings the will contest. The contestant would have to prove that, at the time of execution of the will, there was fraud, duress or improper pressure put on the testator leading to a will that unfairly excludes or limits gifts to the contestant or other beneficiaries. If the will itself was forged, obtained by fraud or improperly executed, the will could be deemed invalid, and either a prior will or the laws of intestacy would control. If the testator was not mentally competent enough to know what she was signing, the contest could be for lack of capacity.
Also, most states have statutes that, in case of murder, prevent the killer from receiving anything under a will. In addition to the above grounds for invalidating a will, in some states, for example in California, it is possible to claim elder abuse or financial elder abuse.