Good question! There will be no such thing as a general check, because -- for any identity -- the client software has to perform a check that a human would agree means something. For example, what does it mean that you own a certain blog? How would a person confirm it? Well, at first glance it might mean that you have the power to post a message there. But someone else could do that it in a comment, and so that wouldn't work with Keybase. So any given identity check has to match some human definition of what it means to have that identity. And it has to be publicly auditable.

With twitter, it's the ability to post a tweet under a certain username. With owning a tumblr account, it might be something similar. With your known StackExchange profile it might mean posting a statement in a specific part of your profile. And so on.

The common thread in each case is (1) that you post in a place where only your identity can, and (2) what you post is a signed statement claiming a connection among three things: (a) your keybase username, (b) your public key, and (3) the identity on that third party service. (The third one is necessary so it can't be moved elsewhere.) Note how twitter and github's are totally different, but achieving these three things.

We will build out this list of identity checks, hopefully making all kinds of them easy to do. Everything from proving you own a domain to having a tumblr or reddit accoun. The definition of those checks will all be publicly reviewable, both in the spec and in the client, which is what checks them for you.