Hi Evans,
First of all I suggest having a look in SAP Help - BI Content for the respective DataSource. However, it is not always documented in a detailed way and sometimes source table names and fields are missing.
The next step is to have a look at the DataSource using t/code RSA2 (in the SAP source system). Here you can find the Extraction Method and Extractor. If it is a Function Module Extractor, then you can use t/code SE37 to have a look at the extractor. Here you can analyze the source code on table names. E.g. look for TABLES statements and SELECT statements. If it is a Transparent Table or View, use t/code SE11 to analyze. If it is a Domain, then also use t/code SE11 to analyze.
A third option is debugging the extractor. This can be a time-consuming process.
Besides this there are sometimes interesting SAP Notes with additional information on the DataSource.
Best regards,
Sander