I tank a lot of dungeons and I've learned the cadence and pace of every pull and boss, so I can tell when a boss is taking longer or shorter than it needs to be after countless runs. So when I see a boss going down really fast and look at the threat meter, I can usually tell who is doing the most or I'll just check their gear. I've seen some crazy dps tear bosses apart in record time and I'll make sure I comm them. If our DPS is that good, I'll specifically request the healer comm one while I comm the other. Also, since I've leveled every job to 80 and understand how all of them play, I can tell after only a few seconds of watching when a dps is doing their rotation correctly.