The rule of thumb in genealogy is to verify every fact with three different sources, such as birth certificates, marriage certificates, land records, journal/diary entries, letters, and so on. These types of documents are available through different types of resources, such as county birth, death, and marriage records; court records (land and legal records); clergy records; family papers; and, above all, Census Records.
Genealogy databases like Ancestry allow you to search data that was entered from the census records of many countries, as well as vital statistics records. Often you can also view the original document, which as been digitized and loaded into the database. It is essential that you view and obtain copies of original documents whenever possible, as sometimes errors are made when transcribers enter information from the document into the database.
Below are three local libraries that have resources to help you find this documentation. Searching is free but is not available from outside the libraries.