How Do I Know What I Know?

Going back to the most basic knowledge one can have, Rene Descartes came up with the idea "I think, therefore I am." Because I am conscious I know I exist. Though we don't question our existence in everyday life, if you do it can get pretty confusing. So I say it's better (for this question at least) to stick to more concrete ideas. For example, how do I know simple things like stealing is bad? The easiest answer would be that this was taught to me by my parents and by society. But do all people need to be taught this? Sure, some people would (and do) steal regardless of what they were taught. But some people might naturally see it as wrong. If I had never been specifically told stealing is bad as a kid, I still think I wouldn't have done it. And if I had stolen something, maybe I would have learned from experience and seen the reaction of whoever it belonged to, and I would realize that it made them unhappy, so I shouldn't do it. Then again, there is the question of the concept of property. In order to steal, things have to belong to someone. If we weren't taught the concept of property, we couldn't learn through experience that you shouldn't steal because you don't believe the object belongs to them, regardless of what they believe.
Another interesting question is how do I know my parents really are my parents? Most young children, even if adopted, don't question this. They believe that the people who raised them are their parents, much like how a newly-hatched duckling believes the first living thing it sees is its mother (whether that's really true or not is another question). As we get older maybe we do see similarities between us and our parents, either in how we look or act or both, and that seems to confirm it. But if you think about it, if you're 'parents' have been saying since day one you have dad's nose, or mom's eyes, you're bound to believe it whether or not it's true, as long as it's not too long of a stretch. I'm not saying that parents have some sort of conspiracy going to make you believe you're their biological child, but it's an interesting way to look at how we get some of our very first knowledge.