4. Back end database(s)
If you have identified that there is one or more back end databases and how the application connects to it, turn your attention to the database server. Use things like:
* Profiling or tracing tools with the client running
* Table design and data to see how well it works and looks compared to data in the client application - and whether access is via stored procedures or ad hoc SQL statements.
* In front end tools, if linked tables are in use you can look at the data source to identify connectivity (especially Microsoft Access).
If a database is in use, try and identify how its data gets populated. It may be entered through the user front end application, but equally there may be another system that uploads data for use in this one. If the latter, then there is another application to look at.
Its worth noting that the database may be on the same computer as the front end (eg if it is Access or SQLite or other file based database).
If you know there is a back end database but not what the platform is, have a look at the files that make up the back end and use something like http://filext.com to see if you can identify the application that uses it.