Hi,
Raf suggested you to check ECC system, T code - SE11, enter your data source extract structure and see there
My suggestion is at BW side,
If you want the fields which are part of data source at bw side then you go to your data source in bw side. Double click on data source and click on Fields tab.There you can see what you need and try to save them into excel.
Another way in bw server:
You can go to your data source and first layer target transformations.
double click on your transforamtions, menu Extras---> Tabulur overview. form there also you can downloaded all data source fields into excel. You need to play with menu options.
Thanks