HI Cariebb, I actually found out accidentally by myself when I came across my adoption certificate in my parents' wardrobe. Ha ha! Initially, I was blur blur because the name was different but after reading it twice, I realised that it was me! My first reaction, besides my pounding heart, was 'wow, my parents, relatives and their friends sure knows know to keep a secret!'. 2nd thought was, "I'm so blessed because I'm not blood related and yet I'm so so loved'. And finally my last thought was, "oh no wonder I'm so much taller than my parents" Ha ha!
Anyway, I kept it a secret for 1 year because I wasn't sure how my parents will react if I told them. In hindsight, that's funny how the roles got reversed! Anyway, when I eventually told my mum, she was so relieved and happy and told me if I ever wanted to find my biological parents I am free to do so but I told her I've already decided not to (since I had so much time to think about it).
Anyway, I'm not sure if my experience is the same for everyone. My childhood and growing years are absolutely wonderful so that could play a part in my reaction too. I know that I would never be where I am today if not for my adoption. So I am very grateful.