How can someone's testamentary capacity be challenged?

You may have a tough row to hoe if you want to prove that your father didn't have the mental capacity to make a will at the time he made it. The probate court will assume that he was of sound mind, so it will be up to you to provide convincing evidence otherwise.

Forgetfulness, by itself, isn't enough to prove that someone was of unsound mind. You'll have to show that your father didn't know who his family members (the people to whom it would be expected that he would leave his property) were, didn't know what he owned, didn't realize he was signing his will, or was under someone else's undue influence.

If his will was properly witnessed--that is, two adults who don't inherit under the will saw your father sign the document--their testimony will be very important. If they state that your father appeared to know what he was doing, you'll have to come up with a way to show that they're wrong.