How does one find out if they are adopted?

My birth certificate raised a few questions that I had asked my dad about. Mostly dates. My dad was very helpful and told me I was adopted. My adopted mom(my adopted dad's exwife) could not have children. So they have adopted me in 1964 and my brother in at 2yrs old in 1968. He said that her mental instability in not being able to have children she distroyed the copies of the adoption papers they had in their own records, for she wanted live a lie that she gave birth to us. She was institutionalized many of time during my early childhood. The happiest day for my brother and I was the day he divorced that nut. If it wasn't for my dad and my stepmom, I would have never gotten this far. What makes my brother and I laugh, is that she looks nothing like us. Maybe someone with compassion and living the lie, will come forward and speak the truth. I believe they are being self centered and only care about themselves. I do not like liars and if a person lies, how can you ever trust them again. Good luck and God bless in your search, Judi