How can you ‘tell’ if someone is a Christian?

Specifically, how can you ‘tell’ if someone is a Christian?

To summarize: can I tell if someone is a Christian just by observing them?

If I meet someone, I often notice the ‘tell’ signs such as: their face lights up as they talk with enthusiasm about their faith.  You can hear it in the way they talk and what they talk about. After all, everyone talks about those things (and people) that are the most important to them.