How can you tell a person is a true Christian
I've been toiling with this question in my head for a while after a conversation I had with a preacher friend where we were discussing people leaving Christianity, and that being evidence that the religion is not true. He sited the bible (not sure what passage) that some people are "with the flock" not "in the flock" referring to the flock of Christ. I pointed out that this is a No True Scotsman fallacy but after some more conversation and thinking I have come to the conclusion that it isn't the same. Is there a name for this fallacy, if it is one?
I was wondering if the only way that a christian can tell if another is a true christian is through either divine revelation or when the other leaves the faith.