This doesn't really answer your question of "how do you know", but perhaps I can offer some insight to why some tanks don't like to do big pulls. It may be due to input overload and the resulting discomfort. I rarely have this in FFXIV but I'll use two other games as an example.
Path of Exile is an ARPG where the endgame consists of "maps" - items which open up a playfield with specific parameters when used. Close to the max level EXP gains become very slow, and the general consensus (at least when I last played) is that the best way to get EXP is to do repeated runs of particular types of maps very fast. I tried that once, and while I survived the attempt, I constantly felt like I was not in control of the situation, that at any moment a surprise monster pack could get me killed. So I opted to just spend 7-10 minutes to complete each map in a way that lets me consider each situation and make me feel safe, instead of the 3-4 minute runs that some players do.
Deep Rock Galactic is an FPS where you mine minerals and fight monsters. On higher difficulty levels more enemies appear and they hit harder. If there's enemies on several sides, or I'm in a too tight space, I can no longer process all of the information I'm given to find a safe escape route. Sometimes it's hard to find even a single flying enemy which keeps shooting me. If the difficulty is high enough that usually results in enough damage to kill me. Fortunately the difficulty level is selectable so I can pick one I'm comfortable with.
In FFXIV this sensory overload problem is less severe because all the information about the fight is in front of me and enemies mostly don't appear out of nowhere. Having a lot of enemies using AoE on me can still degrade my performance. My focus shifts to avoiding the AoEs and attacking goes on autopilot. That means I'll mostly use the two basic AoE attacks and forget more fancy attacks with cooldowns. I'm pretty good at avoiding the AoEs though, so as long as the healer and dps are up to the task of keeping me alive and eliminating the enemies it's fine.