On the surface, it may seem like an easy question. Data Owners typically think this has an easy answer.

If their data followed a very simplistic model, then it would have an easy answer. Just “select count(*) from table;” and report the value.

Unfortunately for Data Custodians, the data is often organized in relational data structures. In my case, we almost always buy a product and are not provided or not allowed to see schema documentation. So, this question requires making judgment calls.

1. What kinds of data do I have? Hopefully, the tables have decent names that are meaningful. Learn how to use the product and the basic concepts of what things are will come with it. Assuming the tables are meaningful, then a review of the table names will suggest where the likely important data is stored.
2. Research elsewhere if necessary. People blog about what they know. They ask questions in forums.
3. Experiment. Change the value in the application and look to see whether it shows in the table.
4. Count. Get the number of records for the relevant tables.