You need to look at the actual source code.
A DSPF should have source in QDDSSRC (or whereever the development team stores the source). You can then look for the line# & Start position from the screen of the input field. When you find that in the source, you will find a field name.
Now you need to go to the RPG or COBOL or whatever source and find out how that field is being populated or outputed.