What are some signs of an abused dog adopted into a loving family?

I have two male boxers, Rocco and Joe. We have had Rocco since he was a puppy. Both his mom and dad came from Germany and had Rocco's litter in America. Germany is where the Boxer was originally bred. Joe is our newest boxer. He was abused in his last two families before we took him into our home. Chad, the guy we got Joe from is my dad's friend, we just recently found out that he had abused Joe too. That tore our hearts. Chad came over to our house one day and Joe instantly flinched and ran from him. I had to take Joe upstairs and comfort him until Chad left. Chad's girlfriend comes over next door a lot and when Joe is outside, he see her and runs to her. So she probably didn't abuse him. Joe is starting to do better and coming out of his shell. He gets along great with Rocco. They never leave each other's sides. They play all the time and share the same little bed. He still is a little jumpy, my dad yelled one time,not at the dogs, but because of something else, and Joe went into his kennel even though he wasn't in trouble. He is doing a lot better though, we are all seeing it.