The first thing that you need to know is that there are sources that are primary and then there are sources that are secondary. Primary sources are those that are created soon after the event has occurred and secondary sources are those that are second hand and are based on evidence that is gathered after the event has taken place. By definition primary sources are more reliable than secondary sources since they are created at the time when the event is taking place.

Some of the primary sources include a midwife's journal entry describing the child she delivered, the christening ceremony details recorded by a priest, details of the rites performed by a parish or the testimony of a mother describing the birth of her children. Examples of secondary sources include general descriptions of events that have taken place earlier.

While primary sources are relatively more reliable than secondary sources, there is a possibility of an error in the dates or the names in both kinds of sources. The idea is to try and get multiple sources to get to the same data and then see if there is a clash between the sources or not. In case there is a clash, information from primary sources generally takes precedence. For example if census data states an age that does not match with the birth record, it is the birth record that will take precedence over the census data for information about the date of birth of the individual.

Do remember that when you are verifying a document, all aspects need to be verified equally carefully.