My mother-in-law passed away, We have a will, beneficiaries and don’t need probate. How do we cash a check in the name of her estate?

You don't.  A Will has no legal effect until a court admits it to probate.  Probate is proving that the Will is the Will of the person who died and granting authority over the deceased person's assets to the person named as the personal representative in the Will.  Contact a probate attorney who practices in the county in which the death occurred.