Okay, if you are interested, what I was really getting at was not "what" people believe, but rather "whether they actually believe what they think they do." And very, very often, I think the answer to that is "no!"
So how do you find that out? Often, fairly simply. An example: I believe that little spiral of metal on the top of my stove, if it is glowing red, would be a really, really bad thing to rest my hand on. Trust me, I believe that, because as someone who loves to cook, I've burned (and cut) myself often enough to give me a some confidence my belief is correct. As a result, I would never, ever, unless coerced in a way I could not control, put my hand on that burner. That's belief! That's visceral, for real, heart-felt belief.
So try the same test with any other supposed belief. If a Christian tells you he profoundly believes what Christ said when he said "love others as I have loved you," and then ask them what they fell about the gay couple down the street, you might find that their supposed belief doesn't actually align with their behaviours. So then you have to ask yourself, which is the real them...their stated belief, or their treatment of those neighbours?
Here's a trivial challenge … try looking at Pat Robertson, Jerry Falwell, or any number of other religious leaders through that lens, and see if you conclude that they really believe in the Christ they pretend to know.