When executing a fit/gap analysis (are all necessary R/3 fields already extracted and transferred to BW), table RSOLTPSOURCEFIE can be used.
Within the selection screen you can enter, for example, R/3 field MATNR if you want to know in which datasources MATNR is used.

The screenshot below shows the overview of all datasources which use R/3 field MATNR:

To see, per datasource/transfer structure, to which infoobject the R/3 field is mapped, table RSTSFIELD can be viewed.
In the screengrab below you can see that R/3 field MATNR is mapped to infoobject 0MATERIAL within several transferstructures/datasources.