When I was in working on this stuff they went to my birth hospital and checked my fingerprints and footprints on the birth records. Then they verified my parents.
When my second cousin went into military intelligence in the early 80s, I had a couple of uniformed officers show up at my house to ask questions. They asked how close we were, did I trust her, did she have lots of boyfriends etc. They also asked if I had had recent contact with a friend of my parents who was a civilian psychologist at Wright Patterson AFB and did my cousin know him.