It is always difficult and painful when a close relative dies. What can make matters worse is receiving a copy of the will and feeling certain that the contents were somehow influenced by an external party. This is both a matter of making sure that the people truly owed something are given what they deserve, but more than this, it's about honouring the true wishes of the deceased without outside influence.
To the untrained eye there is not much that separates legitimate influence from undue influence. If the deceased had a strong, albeit unhealthy relationship that pushed them in a certain direction, that doesn't count. In the eyes of the probate court, coercion must have been involved. There are certain things, however, that a probate lawyer can use to do their best to prove the case.
Prove that the deceased was in a weakened state. If a lawyer can prove that the deceased was more susceptible to persuasion as a result of their physical or mental decline, there is reason to believe their hand could have been forced in a direction they did not choose. Fortunately, this is a fairly straightforward proof that can be legitimised with medical records.
Gather proof from witnesses. The simple fact is that there isn't always going to be a witness that you can lean on, but scour as hard as you can because a compelling testimony in the witness stand can very often swing a case. This witness cannot simply be someone who suspects something, they need to have actually witnessed wrong doing or the weakness of the victim. A doctor who can attest in person to a decline in mental health can be very valuable, and of course, anybody who witnessed a threat placed upon the victim regarding their will.
Prove that the other party took advantage. It has to actually be proved that the other party actively took advantage of the victim in their particular relationship. This proof needs to be along the lines of an ill person making a will change at the request of a third party, or of making an investment at the request of that other party.
Move quickly. Once all of the assets from the will are allocated, it makes it harder to contest the will. For this reason, raise a flag at the first hint of something looking suspicious, and do consult with a probate lawyer as soon as possible to ensure you are doing everything you can to protect the assets in the will.