How to find what mineral rights a person may own

This inquiry stems from an unsolicited offer made to my father, who was identified as a potential heir to mineral rights originally owned by his late wife’s grandfather. Because my father is elderly and the individual in question would be my great‑grandfather, I am assisting him with this matter. Everything appears legitimate so far, but I want to conduct my own research to better understand the situation.

We have not received a formal offer, but the company informed us that the mineral rights are located in Township 3 South, Range 64 West, Section 27. Is there a resource available that would allow me to determine whether the mineral rights are still titled in my great‑grandfather’s name?