Some of it's experience, some is research, some is informed hypothesis (aka: educated guess).
Quoy's are probably the best parrotfish for a reef - but that doesn't make 'em a sure thing. I've had one go for several months (lost to what I believe to have been brook, during a "fallow" exercise), while another never fed in my care. I've had the one I've got now for a few months, and he appears healthy. I've heard of them living to a few years - but I've heard of them appearing healthy for six months or so, and turning up dead on the sand one day. At this moment, I would have to consider them a somewhat risky proposition.
A blue-throat in a 120 might work. I tried one in a smaller tank (65 gallon, while the 220 gallon upgrade was sitting empty on the other side of the room), and the fish darkened to female colors - while all of my shrimp disappeared. It is a fish that gets to decent size and likes to eat and move, so the longer your 120 is, the better, I suspect.
~Bruce