The problem you are having is that you do not have a logical model of your database. For now you can download a trial version of PowerDesigner.
And use the tool to reverse engineer the database you have. What you are really interested is the relationship and constraints among various tables, the primary and foreign keys etc.
In the absence of this tool you can look at a given database by examining tables in sysobjects, syscolumns, sysindexes, syskeys and sysconstraints. However I strongly advise you to use use a modeling tool like PowerDesigner for this purpose.
This was first published in October 2004