How can a forum provide a trusted environment where all members can reasonably trust the identity of all other members?

I appreciate that this is not a concern for many (case in point: HN). However in some circumstances a forum moderator wants to be reasonably sure that all members use their real names, and are who they purport to be. For example: private forums serving a profession. Anonymous accounts would not be permitted, and usernames would have to reflect the user's actual name -- or their account would have to declare it to any forum member who went looking.

What is your identity verification process, and what lessons have you learned?

Obvious online providers are the ubiquitous Facebook login, however many people distrust this. LinkedIn offers an alternative, but suffers from lower uptake.

What other providers are feasible? Does anybody here use PayPal for this purpose?

There is no perfect solution -- I'm interested in experiences, workflows, and lessons learned.

This question isn't in support of draconian policies [1], it's about how to provide an online environment where identity can be trusted by all participants.

[1] http://www.computer.org/csdl/proceedings/hicss/2012/4525/00/06149194.pdf