>I currently check for the existence of a specific file in a specific location to determine if a "module" is installed.
>
>RETURN file(_screen.mvApplication.DataDirectory + "PEOPTION.DBF")
>
>How can I accomplish the same thing in SQL Server?

In addition to other responses, these are two statements that executed through SPT or ADO will return a list of tables on the current database:

SELECT * FROM INFORMATION_SCHEMA.TABLES
..or
SELECT * FROM SYSOBJECTS WHERE TYPE='U'

Probably the most direct way of checking a single table is through Mike Levy's response. HTH