I can’t speak to the Marine Corps, but I can tell you that the Army has multiple screening processes for returning vets. I’ve lost count of the number of social worker/psychologists I’ve talked to upon redeploying, and there wasn’t even anything wrong with me!
Regardless, it’s good that the Army does these things to catch the actual at-risk guys. Hopefully the Marines do the same thing.
I’m speaking from an Army background myself, having taken training on how to recognize various combat injuries. I don’t know about the other services, but I assume they have similar programs.
I had an NCO who told me he had PTSD. He could not walk into a crowd without feeling very paranoid.