I can generally tell what people believe by how they treat one another as well.
Those who often call other people unsaved more than likely don't understand grace for themselves.
Even in Scripture the writers continually said, "Hey believer this is wrong, but we believe THIS about you..." Good things not evil things. They saw the good in people, they saw God in people.
The writers of Scripture do continually advise people to ensure they themselves are saved though. Examine yourselves to see if Christ is in you... the key is understanding that we are God's temple. And because God lives in us His fruit WILL be seen in our lives. This isn't to be saved. This is because we are saved. It is in our struggle that we find we need the Comforter to remind us.
Paul even went so far as to say we know no one by the flesh. If anyone is IN Christ they are a new creation.
Are we seeing God in other believers? If not, the problem isn't other people.
Another way I can tell what a person believes is how many times they refer to love. The chief Scriptures we interpret the Bible through is more than likely who we believe God to be. But Scripture overwhelmingly teaches that it is by love that we know God.
What is our heart saying about our beliefs?
James 1:26 If anyone considers himself religious and yet does not bridle his tongue, he deceives his heart and his religion is worthless.
If someone doesn't know how to love, I usually don't believe they have much to teach me about God. Because they scarcely know Him.