Usually watch the rest of the party. Pre-50 before everyone has good cooldowns or AoEs mass pulling can be risky. But beyond that look for a few things.

Are your DPS doing single target or AoE? I've ran into plenty of parties where we can have 8+ mobs and a dps insist on their single target rotation, that can make big pulls futile.

Is your healer helping with AoE? Not every single pull will allow it, but usually all 3 healers have cooldowns along with a tank's cooldowns to at least allow them a little bit of dps, WHM especially since their AoE is effectively a damage reduction with the stun effect.

Paying attention to gear is also important, the 71 dungeon for instance I've seen a ton of tanks try the first few huge pulls with bare minimum ilvl, it very rarely goes well unless the healer and dps are on point.

It's not always the same thing to blame, you just have to keep an eye out, if your healer isn't confident or is the half afk type just stick to small pulls