It was my understanding that there is an original birth certificate out there that shows the biological parents. That information may be sealed but it exists. My DH had a son given up for adoption. There are records out there that show he was the bioligical parent, because they had to go to court to adopt the child. The adoptive parents were not official at birth.
I think it may vary from state to state, and of course laws change over time. Assuming that all or most states now retain some form of original biological parentage record to the extent that the information was available, it's still not the person's official birth certificate, once it's been replaced with the "legal parents" edition. And I believe that at least most states don't put anything on the new one to indicate that it's not the biological parents who are listed, and that if an adopted child isn't told that s/he is adopted, there's no way for him/her to tell from the official birth certificate.