Ya gotta do a title search. The leaseholder will certainly do one, but you can do it yourself and see if you're entitled to any royalties or not. What it boils down to is whether or not your father's mineral rights were deeded to anyone. If not, they get split among his heirs. At least that's the way it is in Colorado, which is the only state for which I have any practical knowledge.
So you need to go to the court house and do a title search on the property in question. You need to find the deed that transferred your father's 20 percent to him and any subsequent transactions. You'll also need your dad's will and any other estate papers proving you're entitled to a share. At that point, you should have a pretty good idea if you have a claim, then you can decide if you want to get an attorney involved. Having done the research yourself should save you some legal bills.