I usually take their word for it. "Christian" is, for all intents and purposes, a self-described identity.
Ah, but you hold a preconceived notion of what that means, what it entails. You do not make room in your presumption for the possibility that the individual self-describing as Christian might just believe that Allah is the only God and Mohammed is his Prophet.

Would you really accept such an individual as Christian? Don't labels become meaningless if they have infinite scope?

Do you fire a series of questions at someone regarding his/her beliefs, and the consistency of those beliefs, when somebody says he/she is a christian?
I'll bet he smirks internally and thinks, "I'll believe it when I see it."