Most calls start with something you can see from the yard. The mortar crumbles when you press a finger to it, sand gathers at the base of the wall, a stone rocks in its bed, or a damp stain won't dry. Those are usually signs that water has been getting into the joints, and the sooner the joints are sound again, the less of the stone itself we have to touch.
The price mostly follows how much of the wall needs new mortar, how deep the old joints have worn, and whether stones have to be reset or replaced. The schedule turns on mild weather, since mortar needs it to set, and on any permit the job needs. My written estimate lays out what your wall needs and why, so you can weigh it before deciding.
Wisconsin ties building permits for one- and two-family homes to the state's Dwelling Contractor certification, which a contracting business holds in order to pull them. If you're comparing masons for work that may need a permit, it's a fair thing to ask about, along with how each one plans to match the mortar to your wall.