Check I.D. ?
Why would I check my dad's ID? Or my sister's? If I don't know who those people are already, I should be declared incompetent and put on the "prohibited person" list myself !
Seriously, for occasional in-state transfers of common firearms (not NFA-regulated weapons like machineguns, etc.) I think all you "HAVE TO" do is sell it to anybody who wants to buy it who doesn't appear to be disqualified from possessing it. Only if you see or hear or otherwise know something that would reasonably give you concern that the person might not be qualified would you then have a duty to ask specific questions, check ID, get them to sign documents, etc.
But I've always done a lot more than the criminal law would require, when selling one of my firearms to a stranger. I keep in mind the prospect of civil liability, too. Bottom line: Don't be negligent. Behave reasonably and responsibly. To me, that means talking to the person a little bit to find out who he is, where he resides, what he wants my gun for, what kind of training or experience he has with firearms, and whether he is ineligible to buy a gun for any one of several reasons.