You can tell what type of database the Access front-end is from it's file extension. Prior to 2007, there were four common ones, .mdb and and .adp, and .mde and .ade which are compiled versions of the other two respectively. Access 2007 introduced a new format called .accdb, and it's .accde compiled version. The fact that the Linked Table Manager is greyed out is odd unless it is the .adp/.ade format. In a compiled version, you cannot view the design of forms and reports, so it appears that you are probably not dealing with a compiled version. If it is the .adp format, you should see stored procedures rather than queries when you expand the Navigation Pane.

There is another possibility however, that being that all of the SQL Server tables are accessed using ADO in the VBA modules. That makes it considerably more difficult to sort out how things are being done if you haven't had much experience with Visual Basic for Applications. If that is the case, you should see a connection string at the beginning of the module behind forms and reports that establishes the data source.