Tuning? or Truing stand? I'll bet truing stand. I've always left the freewheel/cassette on the wheel unless removing it for service/cleaning, etc.
Perhaps if you're judging tension by pitch, tuning stand might be appropriate? In any case, it doesn't matter if the cassette is on the wheel or which side it is on, though I always put it on the right so it's easier to keep straight in my head. You can flip sides to check dish.
I've reread the post several times after my reply and honestly make no sense out of it but it was fun to read.
Yeah I use a dish tool but guess this person is talking more about checking the "trueness" of the wheel and is new to it.
Just be sure to check the tension of all the spokes before starting any truing project...if not done correctly you can wind up really messing up.
If you're not sure do a search on google. I'll bet there are quite a few videos on wheel truing on a bike, stand, etc.