My brother and I have red hair. My mom has blonde hair, and my dad had black hair. We knew we weren't adopted, but when we were little, we had that little inkling of wonder, especially when people would always ask where we got the red hair from.
I never wondered about that. I have my County and Hospital birth certificates showing who my parents are .. kind of difficult to doubt that. I look a lot like my Dad too. (I hope you can find out for sure at some point, if you have your doubts.)