Mineral Rights offers in Caddo and Grady County

Hi, new to the forum. We have had mineral rights since 2008 when our mother passed. We have seen many companies come in and out of Caddo and Grady County. We had significant offers to sell mostly in the latter 20-teens. Then that all stopped around 2020 and the checks have diminished as well. Recently, we have started to see Continental pick up the activity and offers to sell have started again. The checks are about the same for now. We expect that to change. Where can I go to find reasonable estimates of the mineral rights value? Any help is much appreciated. I have no way to assess if the offers to sell are fair and reasonable.