A good squirrel dog will tree a squirrel using it's eyes, ears, & nose. Mine will tree if it sees or hears one. It will trail a scent on the ground to a tree. But best of all he can wind a squirrel that's up in a tree from 50-75 yards away. He'll run through the woods with his nose in the air until he zeros in on the tree. This is pretty exceptional for a Feist, but most Curs are good at it.
Curs generally have better noses, and Feist generally have a keener sense of sight & hearing. It's a terrier trait.
There are a few fellas in the Wi., Il, & In. areas that breed good Curs & Feists.