Many adoptees find with age an increased interest in reuniting or meeting their biological or birth parents. There's no sure-fire way to find birth parents, but many adoptees may want to begin by asking their adoptive parents for any information or insight they can give to help the search process.
Each state should also have a search registry where adoptees and birth relatives can register, mutually consenting to the release of identifying information from the adoption records. It's also possible for an adoptee to petition the courts for a confidential intermediary to gain access to the adoption records and to facilitate communication between the two adoptive parties.
Agencies are another way to go about learning how to find birth parents. Some agencies may offer search services to adoptees for a fee. Other ways to find birth parents include online registries, organizations like Internal Revenue Services (which would require knowledge of a birth parent's social security number), and other search agencies or private investigators.
Many online registries are simply databases, through which a searcher can look through all other registrants for a possible match. This isn't generally the best route, as it require the person being sought to have registered with the online database as well. There are also many volunteers, such as Search Angles or the Volunteer Search Network, where adoptees can turn for extra, free help looking for their birth parents.