If the original birth certificate has been marked adopted, then the adoption would be deemed a "legal Adoption", and both birth certificates would be available from Groni in the normal application process, one from the birth registers, and one from the "adopted childrens register" which was compiled in 1948 but included adoptions from 1922.
The other route would be to aquire baptismal records, i have one marked " adopted son of" but a through search has not revealed a legal adoption, and that was as recent as the thirties. So like all events, some adoptions were only assumed to have taken place.