The law says that, if a step parent wants to adopt their spouse's child, then the biological parent has to sign off on all legal rights to the child. Or if the parent can't be located, an announcement used to be put into newspapers and letters sent to the parent's last known address. If they never respond, then after a certain period of time, i do believe the Courts will grant the adoption.
This is how it was always done through the legal system. If your adoption didn't go through the Courts, then it's not legit. You can probably get a copy of adoption papers from the Clerk of Courts in the county where the adoption took place, if there was one at all.
If there was no legal adoption then you're not using your legal name.