Do I have the right to research my Passed Fathers financial history to see if his current wife changed a policy that I was the beneficiary on

My father with dementia passed away last week. His (3rd) wife had durable power of attorney since 2014. My father told me that I was the beneficiary on his 401K. His widow tells me there is nothing for me and all accounts were put into both of their names. If she put the account in question into her name after obtaining POA do I have a legal right to dispute that it was done in my fathers interest?