https://torrentfreak.com/how-investigators-use-osint-to-track-down-iptv-pirates-220710/
By checking message boards for all the " Ger my IPTV, it's brilliant" threads
I believe some channels put unnoticeable markers in their content where the investigators can view which sub/station that was being broadcasted from. Beyond that I'm not sure.
Loose lips sink ships.