In a way, its for the person to tell the difference not a 3rd party. However, experience tells me that I'd tell the difference by whether someone felt that that was something missing from their stride.
Ask around - plenty to choose from
You can start with me - nothing missing.
I've heard folk suggest (though I'm not so convinced by this) that one can tell by the determination that someone has to diss Christianity - or religion as a whole.
Aside from the very obviously
ad hoc madey-uppy nature of that by people who don't like their batty and wholly unevidenced beliefs being challenged or criticised, people 'diss' Christianity and/or religion as a whole because they dislike it - its silliness, its puerility, the credulity and servility and guilt it fosters.
And of course because it's just plain daft, and wholly unworthy of sensible grown-ups.