It does depend on who you ask. The most basic requirement is a confession of faith. As in, for example, the Apostle's Creed.
Personally, I do believe that most of the Bible is a metaphor for greater wisdom; however, I asked to be baptized at age 7 and I have been studying for decades. I think a foundation in text and interaction with other believers in serious inquiry is vital (i.e., Sunday school). I think once you have done some real work, if you can depart from certain widely-held beliefs, whatever they may be, in your particular denomination, that is righteous. I think you should know what you're doing, though, and that does not mean willy-nilly on the basis of nothing up and deciding that whatever you feel like is how it should be.
That is the truth for me. Your truth may be entirely different and whatever it is, I respect it as Holy Spirit instructing you as is appropriate for who you are where you are.
So how do you tell, exactly? Who's to say who's actually a Christian and who isn't, or which belief alterations are justifiable and which ones aren't?
And doesn't this cause a few problems with the whole "salvation through faith" thing? If we can't come to an agreement on what sort of faith warrants salvation, how do we know which variant to follow?
If you could be anything you want, I bet you'd be disappointed--am I right?