Finding Datasource from fieldname

Hi Experts,

I am new to BW and have a question. Users come asking with field names from the tables that they want to see in the data extractors.

How can I find out if that particular field is already available in the any of existing data sources ? Is there any table that I can see to find out if that particular table field is already being extracted. Pl. remember that we do not know the datasource name .. so how do I find out that ?