When I was a Christian, I would have answered that we don't know who is a Christian. Only God does, and He will admit them but not others claiming to be Christian into heaven. That's obviously not a very useful definition for an atheist.
Like many others, I don't have a category of authentic or true Christians. There are only Christians and non-Christians. If someone believes that Jesus is the only path to salvation in an afterlife, they're a Christian. That's my only requirement to call them one, and I virtually never ask if that's what they believe. I assume it, so in effect, anybody that claims to be a Christian is one to me as well. And that's the definition people are using when they say that there are 2.38 billion Christians in the world. Somebody extrapolated the result of surveys and censuses where people self-identified as Christian to come to that figure. I am aware of only one human being, a very well-known person, who has claimed to be a Christian, but whom I don't count as one - somebody who I am sure is lying about holding even the minimal core belief I suggested. So, the twin definitions provided yield almost perfect overlap. Meeting one means you will meet the other.
There is no other doctrinal test, as some Christians have for excluding other Christians (by my definition of Christian), such as whether they have a pope or they baptize by sprinkling or immersion. Theology certainly doesn't matter to the unbeliever apart from a few essential tenets which belief bleeds into their lives, such as that Yahweh (and presumably Jesus as well) hates gays, atheists, and abortions, and those things aren't relevant in deciding who is a Christian except by Christians.
Also, there is no behavioral test. Having no doctrinal test beyond what's described above, and no behavioral test at all, there can be no concept of Christian in name only or not a true Christian.