Another test of a suspicious squirrel is to simply observe how they hold their nuts.
There was a squirrel here for a couple of years that had, uh, nut problems. He'd lay spread eagle on top of the kids' swing set to warm them or he'd cool them by laying spread eagle on the step rail under the porch. I don't know what that meant but it was definitely suspicious of something. You want truly suspicious? There's a squirrel in the yard now who uses sticks to dig. Freaky, freaky.