The process of challenging a will (under rule 16 of the Massachusetts Probate Court Rules) begins when an interested party files an affidavit of objection that spells out "specific facts and grounds upon which the objection is based."
 
Under Massachusetts law, for a person to have testamentary capacity they must--AT THE TIME THEY EXECUTE THE WILL--be able to understand the nature of the document they are signing.  In other words, they must know they are signing a will.  They must also be able to understand the nature of their property (what they own, what it is worth, where it is located), and be able to understand who their potential heirs are.  The burden of proof belongs to the person opposing the will.
 
In cases of undue influence, the person opposing the will has the burden of showing that some person had such control and influence over the party who executed the will (the "donor") that they were able to substitute their wishes for those of the donor.  Courts have described this phenomenon as a person exerting "influence which, by force, coercion or over-persuasion destroys the free agency" of the donor.
 
Because there is rarely any direct evidence of undue influence, the finder of fact must make inferences from the circumstances.  Here are some examples of situations that may create an inference of undue influence: (1) Was the donor in poor physical or mental health?  (2) Was the bequest "unnatural" or unusual (such as leaving all property to a health aide and leaving nothing to the children)?  (3) Did the person receiving the gift or bequest have an opportunity, because of a confidential relationship or because of his constant or unattended presence with the donor, to influence the donor?  (4) Did the donor's will run contrary to the long-held and stated wishes of the donor?  For example, did she always say she wished to leave her house to a specific charity but, instead, leave it to her live in nurse?
 
As you can see from these few examples, each case will be very fact specific.  These cases tend to be difficult and technical, so I encourage you to bring your concerns to an attorney with significant experience with probate law and litigation.