I don't have a system in front of me, but here's how I would approach the problem:
1. Execute the standard SAP transaction that shows you this data and see if the field being displayed gives you a hint i.e. show the System > Status > Technical information for the screen, and have a look through the program too.
2. Run SQL trace / Performance trace for the transaction and look to see which database table are being accessed - you can also drill down into the ABAP code for each select to see how SAP are doing their selections (e.g. there may be a function module you can call to return the results).
