You will need access to Siebel Tools to look at the joins (1:1 M:1) and link (M:M 1:M) objects that define the SQL joins (inner or outer) generated.
You can generate reporting to track this information, basically the column ROW_ID is nearly always used as Foreign Key.
The data model is also available online, what version are you working on ?
Have a look on Oracle MySupport to
<li> Siebel Data Model Reference, Version 7.7 [ID 566251.1]
<li> Siebel Data Model Reference, Version 8.1.1 [ID 557187.1]
<li> Siebel Data Model Reference for Industry Applications, Version 8.1.1 [ID 557189.1]
Any particular reason you're going directly to the tables for BI reports instead of using the Siebel/BI Integration web services? I just finished setting up Siebel 220.127.116.11 and BI Publisher, and found that using the web services is the best method of running reports.