How do you find out if my boyfriend was screwed out of his share of grandmas estate due to being incarcerated at the time of her death?

You will only find this out if the Will was admitted to probate, of if the decedent's land was transfered to someone after his/her death.  In both cases, you have to start at the county clerk's office (in some counties that will be 2 different offices).  Tell them that you'd like to check on a probate filed under the decedent's name, and at land records ask them for copies of any deeds on the land after decedent's date of death.  Those two methods are not fool proof, but they'll give you a good start.  Good luck.

I am not admitted in Washington.

You can check to see if a will was filed in Probate Court.  It would be filed in the county in which his Grandmother was domicilced (lived) at the time of her death. It is a public record and can be viewed by anyone.  If no will was filed then an administration would have been done and a Personal Representative appointed.  You can view that as well.   If you think that there was a will but it was not submitted in to court then your boyfriend has to do some investigating, like contacting the attorney who may have done the Will, etc.  The information his Grandmother told him may lead to something.