My mom died and dad remarried. Dad told me my sis and I were his sole heirs but we've never seen anything in person so how can I check?

An "heir" is someone who takes by intestacy. That means that if a person dies without leaving a Will, the laws of the decedent's state dictate where the decedent's assets go. If he truly means heirs, you would check the laws of his state. A "beneficiary" is someone who takes under a Will or trust. If that's what he means, you need to ask him to see a copy of the relevant document. That is not always an easy conversation to have . . . .