And then there are the litmus tests that Jesus gave you. The one stated most clearly, I suppose, is this one:
By this shall all men know that ye are my disciples, if ye have love one to another.
And we all know it works, too! Nobody but true followers of Christ would love each other.
And then there is the whole sheep and goats thing. Everyone who feeds the hungry, gives drink to the thirsty, takes in strangers, etc. is in. And again, nobody but Christians do that. Obviously.
(Hope your sarcasm detectors are in working order.

)