If you got your hands on their personal threat statistics from their KTM you could use it to get an idea on how their cycles look. Of course this isn't very good data if it's on trash (unless trash is the problem) but if you could get them to save the threat data for a boss fight you could analyze it and figure out their approximate threat cycle.
This could of course be sensitive as it may seem you don't trust their ability to play their class but if you could get that data along with their weapon speed it could maybe answer some questtions you got.
If you get their weapon speed and the number of normal swings and heroic strikes you got the passed time (of course this is offset a little by them parrying and/or flurry/bloodlust). And you got the number of shield slams, devastates, sunder armors and revenges aswell as demo shouts and thunder claps.
I just thought of this so it might be hard to analyze but it could be worth it
