It's pretty easy to tell a healer is being lazy in easy/dungeon content compared to tanks/DPS. You just never see Cleric Stance ever turn-on, and if they do try to do damage without it today it's so nimble that it never makes a difference. It's like you have a painted bullseye if you are a lazy healer, it's so obvious.

DPS sometimes you can't always tell right away. Aside from parsing methods.

Tanks you can sort of tell if they can keep things going without tank stances and using cooldown mitigation. Still, tanks do damage regardless, whether it's optimal or not, you can't always tell without parsing.

DRGs that never have Heavy Thrust up, never use or let Blood of the Dragon. You can tell is being lazy.

BRD/MCH is a little harder cause even if they have Barrel/Minuet up, you can't always follow their DoTs are up or falling off without looking closely, or they keep losing damage by not doing damage on-the-move.