One method of proving identity we already see right now is following and subscribing. Building a following is essentially identical to distributed identity confirmation. Youtube used a similar method for doing this with its ad sense system. It then rewarded people for building larger and larger followings.
Ultimately, a UBI cannot function in such a meritocratic sense because it would be antithetical of what a UBI is supposed to be. But it may seriously by the only realistic solution.
We are going to have to blend social media into the system somehow as a means for people to confirm identities. A UBI will likely be a capitalistic endever in spite of its marxist roots.
I mentioned a couple examples of identity online in the post I linked to above. Trust networks will be hugely important.