I don't know I'm right. When it comes to certainty, I cannot rely on my own self, my own mind, my own logic or reasoning to come to certainty. I am far too fickle for that! My mind is far too sinful to know truth absolutely.
Rather, I know that God is right. When it comes to certainty, like Tim mentioned, we believe in the certainty that God provides through His Word, through the ministry of His Spirit, through His all-encompassing work of salvation to us.
I've really been thinking a lot about relativity in certain areas. The Bible isn't like science, with clearly observable conclusions, and we aren't in an age where God speaks in ways to the church like He did in Revelation for example. How do we know who has it right? We can all have different beliefs on dogmatics it seems, with no way of knowing what the right answer really is. But I'm not implying we shouldn't try to find truth and live according to it.
We know that even in the days where God directly speaks to people that people were disobedient and strayed from certainty. Adam and Eve ate the fruit. Moses struck the rock. Ahab still went to battle. Zechariah had his lips sealed from disbelief. Pilate questioned Truth in front of the giver of truth himself.
I don't think that even if we lived in the age where God spoke regularly to certain people we would be any better off. We would walk through the Red Sea with our enemies washed away by a miraculous hand and still create our own golden calf days later.
The Bible is amazing. Whereas the rest of the world's religions study thousands of books of "holy writ" and its infallible interpretation and interpretation of this interpretation and so on, we have sixty-six short-ish books. Very concise. Very factual. Not full of allegory and mysterious symbolism. Very condensed compared to what others try to write. From these short books comes all good and worthy doctrine. In one sense, it becomes easy to verify against other religions and other beliefs because there's not much source material to work with - you can cross-reference quickly and verify the truth claim without much fuss.
So how do I measure if what I believe is in accord to the truth?
1. Does what I believe about the Scriptures accurately reflect the text? This would be most basic.
2. Does what I believe reflect the beliefs of the historical church? I know the church is not infallible. Yet, if my belief were to differ significantly such that no historical body believed it, I would have to give myself pause as to whether this innovation is truly from God.
3. Does what I believe bring glory to God and elevate Christ? Right doctrine leads to proper worship of God. Proper worship of God reinforces right doctrine.
4. Does what I believe put man and the world in its rightful place? Do I think too highly of man? Do I think too highly of the world?
After all of that is said and done, we will still come to differing beliefs. From there, it's a matter of testing the consistency and inconsistency of what we believe against other aspects of what we believe. When you talk about baptism, you're talking about covenant theology, for example. They go together hand-in-hand.
Patrick and Tim have addressed other aspects of certainty, but there's my two cents.