I am using the following "trick" to determine what episode corresponds to what .mpls file on a blu ray. Usually a season consists of 4 or 5 BD's. I am now in the process of ripping "The Sopranos", which have 4 BD's per season. As an example I'll discuss Season 4. Each BD contains 3 or 4 episodes. Assuming you know the order of the disks, I start with disk 1. I use BDInfo to get the info about the episodes: it says disk 1 contains 00051.mpls, 00052.mpls and 00053.mpls (ordered by file-size, largest first) with durations 57:47, 57:32 and 54:13. Those playlists refer to 00031.m2ts, 00012.m2ts and 00013.m2ts respectively. First I thought it was rather straightforward, with 51, 52 and 53 being episodes 1, 2 and 3, but the strange ordering of those m2ts-files made me uncertain. I wanted to double-check, without having to browse through the content of every other episode!
What I wanted was a reference to the duration of every episode, like IMDB does for movies. This is not easy to find, but it's there all right... Look at: tv.com/shows/the-sopranos/watch/?episode_type_range=1-2 where EVERY episode has a duration (rounded to minutes, but it's good enough). This way you can verify if what you assume is right: it says for episode 1, 2 and 3: 58:00, 58:00 and 55:00. And although nrs. 1 and 2 are the same length, if you do this for all disks you can see the systematics used throughout the whole box. It's always 51, 52, 53 and sometimes 54 IN THAT ORDER. I've checked all episodes from all seasons against tv.com's listing and they all matched up, so now I'm confident I have numbered them correctly.
Hope this helps others. I've been struggling with this thing for a long time, but now I know what to do