When validating & working with data that is merged into one data set frequently it is necessary to verify which features in the output data set came from which input feature data set. Also as part of analysis functionsit is necessary to do analyses based on which input feature data set.
The Merge tool as generally noted in the image below supports merging the data into one data set. From the documentation also generally noted in the documentation it seems that data source indicators are not added to the final output data set since the "field map" does not seem to address the need.... It seems that adding an option to the Merge tool for "add feature data source indicator" would be helpful many different analyses...