The first thing you can do (free) is use her royalty statements to contact the operators. You want the Division Order department. You will need her owner number from the statement. Ask what the net acres and royalty percentage that they are carrying for her. You need those two items and the names of the producing wells in order to get a value. If you already in pay as the next generation, then use your owner number. Ask for a copy of the lease for your files. Sometimes, they will give them to you.
If you are trying to probate her will, there is one path to take. If you are trying to do your estate planning, then a different path from here.