The easiest way to tell is if it was on film or videotape. If it was on film, then it would be in a real place. On videotape it would be in the studio.
Just cannot follow this! I understand that from Series #14 onwards, possibly until onset of HDTV at Series #27 (2006), everything was shot on film.
Mind, this use of film still puzzles me. Film cannot be transmitted - it outputs light, not a transmittable RF signal. I note in Wikipedia for Series #14, "This was the first series to be shot entirely on film, although the complete episode was still assembled on videotape." Why put anything on film in the first place? Maybe need to get to grips with the Alan JW Bell book a bit more.