I used to hunt them in Kentucky.
Best time for me was early in the morning. I tried to get into the woods before daylight.
I would find a stand of hickory trees and just wait. About sun up they would start to cut the hickory nuts. You could hear them and then scan the trees looking for some movement of the leaves in a vertical line as the cuttings fell.
Follow that line to the origin and that is where you will find the squirrel.
Another trick is to look toward the sun in the trees and look for a halo effect you will find when the sun shines through the fur. Once you have seen it, you will never forget it.
Listen for the sound of them bounding through the tree tops.
They are fair game except on the ground or when they get to their nests.
I killed a squirrel by shooting into the nest and I have thought about it for over 50 years. Just don't do it.
Shooting squirrels in the nest is like shooting a covey of quail on the ground.
It just isn't sporting and it should not be done.