It's a really good question, and not an easy one to answer, I think.
Christianity is all about the relationship with God, so I think you would answer the same question the same way you would answer "Is that person a good friend to ____?" What things do you look for?
If you're close friends, and God hasn't come up in a serious discussion between you... then you should probably question why. In fact, you should probably question why
you haven't brought it up if this person is someone you've spent a lot of time with. It's natural for deep discussions to happen in close friendships. Who hasn't debated deep philosophical life issues with their best friends at 2:00 in the morning? (Or maybe I'm just very lucky... those are SO much fun.) That being said, we all know there are people who can talk a GREAT talk... but then their lives don't match.
With that, you can look at what they say and what they do and see how well they match. Everyone stumbles and has weaknesses. Everyone has truth that they
know, but haven't lived up to yet (that's what growth is all about). But you can also see when a person isn't
trying to grow or incorporate the truth they profess into their practical lives. That person may have all the head knowledge in the world about God, but their lives say they don't know Him at all.
Then there are the people who talk the talk, and walk a very careful walk. How do you know for sure? God's love shines through people who have dedicated their lives, thoughts, and decisions to Him. In some ways it's an intangible thing that's hard to describe. There's just something different about them. Like trying to describe light to someone. What do you say? "Well... it's um....
light.... you know... it's... bright... and stuff...." LOL.
Some "concrete" things though are joy in their lives (not the same things as happiness), forgiveness, discernment (instead of judgment), and a balance of grace and truth in the way they talk and treat others.
So... that rambled a bit, but I hope that somewhat answered the question.