Or, do you even care? Personally, I don't. It is a greeting, nothing more, nothing less. It's like saying "How are you?" passing in the hallway. You don't expect the person to answer with anything worse than a "fine." You are definitely not looking for the person's life history. It can't really be that hard to recognize situations where "Merry Christmas" is totally appropriate. A few visual clues might be the bright red and green clothing, the cross necklace, the sweatshirt with Santa on it, or a nativity, etc. Obviously Christian, or at least someone who enjoys and celebrates Christmas for its many secular reasons. On the flip side, there are some obvious non-Christians where a "Happy Holidays" or a simple "Hello" would suffice.
If you truly are offended about "Happy Holidays," then do the rest of us a favor and wear a button that says "I say Merry Christmas!" so the rest of us know. Otherwise, you get what you get. Although I'll be more likely to wish you a Merry Winter Solstice instead.