That's probably impossible to determine. For most controllers at most facilities, shifts change from week to week, and regular days off change every several weeks. In addition to that, they're all certified to work each position, for example, clearance delivery, ground, or one of several local control positions. Where they plug in could be determined by something as simple as who else is due for a break. It's also possible that a controller could be in the tower and never make a transmission, if they were working data, or acting as a trainer to someone else.
The good news is that at BOS, they work either upstairs or downstairs, and don't switch back and forth between Tower and Tracon. If you hear John, it'll be on a tower frequency.
Keep trying and good luck!