All very good questions. I have used many components over after a crash and I just look for obvious physical damage as well as where it was located in relation to the crash( nose, tail, etc..) I had a mid air with my biplane around 75 feet up and landed in a grass field and re-used everything over except the plane
If servo wires were ripped out of the receiver or an extention then I would just use them for parts. Sending a receiver in for testing would be a good thing to do if you doubt it at all. My flying buddy crashed a plane into the woods and did not find it for 4 months. It was sitting upright mostly intact so he reused all the gear in the next one and is still flying it several years later.Would I risk a $1,000 plane on crashed equipment? NOT! A cheap ARF maybe.